Here’s an incomplete list of the biggest things I can remember that were omitted or simplified significantly in the movies versus the books. The whole post is riddled with spoilers so I won’t even try masking them; you have been warned:

The existence of Squibs (non-magic children of wizarding parents; the reverse of a Muggle-born) is never explicitly stated, though the characters of Filch and Mrs Figg (both Squibs) are present.

Probably the largest single overt omission from the entire movie series was the character of Peeves the Poltergeist. He plays a relatively minor role in most of the books, but the comic relief would have been good in areas, and his role became more important in book 5 and book 7 as he actively fought against the bad guys.

The Sorcerer’s Stone movie is probably the closest overall to the book, as the book is the shortest.

The second movie was shot pretty faithfully. A few things are simplified and a couple things are switched around, but I can’t remember much that was overtly omitted other than:

Sir Nicholas’s 500th deathday party on Halloween night was omitted; in the book, HH&R’s attendance of this party leads to them finding Mrs. Norris, petrified and hanging by her tail, at just the wrong time. The movie uses a combination of Harry’s detention with Gilderoy Lockhart and a chance meeting with Hermione and Ron to accomplish the same effect.

The side trip to Filch’s office early in the book, in which the audience learns about Squibs and that Filch is one, never happens.

The fact that Lupin, Pettigrew, Sirius and James (Mssr’s Moony, Wormtail, Padfoot and Prongs) were childhood friends is implied throughout the movies but they never went into much detail; the third book has a lot of exposition about these four.

The meeting in the Shrieking Shack was cut down a bit, and there were some other inconsistencies (this movie actually added more than it removed in the way of small details, if I remember right, in order to add some time and comic relief on events that play out much more quickly in the books, like getting past the Whomping Willow).

The exposition of Sirius as Harry’s godfather, and that he apparently betrayed his parents to Voldemort, was drastically simplified. Also, the snowball fight was actually a mudball fight, and Harry’s cloak comes askew so Malfoy sees his head floating in midair, meaning Harry has to dash back to Hogwarts to avoid being caught.

The replacement of the Fat Lady with Sir Cadogan after Black’s first introsion into the castle was omitted; this also omits the incident where Sirius actually gets into the Gryffindor dormitory.

Harry also is able to use his new Firebolt in that match against Slytherin, after having it confiscated by McGonagall when he first got it under suspicion of it being sent by Sirius Black (still thought to be Harry’s mortal enemy). The movie ends with him taking it for his first spin, with no question of who sent it or their intentions.

In the book, Harry and the Gryffindor team overcome Harry’s first-ever honest loss in Quidditch during the match against Hufflepuff and Cedric Diggory (in which he fell off his broom due to the dementors, the one scene the movie does show), to win the Quidditch Cup for the first time in ten years by beating Slytherin.

Probably got the biggest cuts in regard to Quidditch when going to movie form; in the book, the Quidditch matches are very important plot points concerning Harry’s weakness with dementors and his learning of the Patronus Charm.

Fudge’s denial and blatant coverup of the events in the maze (including Barty Jr’s ultimate fate) are omitted; the essentials are instead given to the audience in film 5.

There’s more Pensieve scenes in the book than the movie; it’s condensed in the film to one scene that tells you everything you need to know, in the process removing Ludo’s backstory and the original introduction to the audience of the character of Bellatrix Lestrange.

In the book, Harry saves Cedric, but is injured and unable to make it to the cup; Cedric is faced with the choice Harry had in the film, of taking the Triwizard Cup for himself or helping Harry.

Cedric and Harry are much more sportsmanlike to each other in the book; the point of the change of the maze itself in the film was to sorely test that because the movie didn’t have the initial enmity between these characters over their last Quidditch game.

The judges’ ratings are pretty much ignored until the last challenge, and that last challenge is very different (the original maze is much more of a true obstacle course, with a lot of magical beasts and other tricks).

The dragon Harry faces in the book is not chained, and is much less aggressive, but stays rooted on her nest most of the time because she’s a nesting mother (which is the challenge for the contestants, to get the egg away from a very defensive dragon).

Rita Skeeter’s poking around and truth-distortion is hinted at but much reduced; the fact that she’s an Animagus and used her talent to literally be a fly on the wall was completely ignored.

Charlie Weasley’s character is omitted as an acted part; he’s only referred to in film 1, but he has a bit part in both books 1 and 4.

Dobby’s role is cut out completely (the hint to use gillyweed instead played out the way Moody had intended it to in the book, with Neville telling him after being given a book on Herbology by Moody), as is the side story about S.P.E.W. and Winky which feeds into a much more complex story concerning Barty Crouch and his son.

Harry and the Weasleys are in the Minister’s box as well, alongside the Malfoys, instead of in the upper deck.

We get scenes from the World Cup, but we never actually get to see the game being played. This is in keeping with the general reduction of the role of Quidditch in the films compared to the books.

Ludo Bagman is just a big red herring in the books and is absent from the films. Because he’s not there, the Weasley twins take over the undertone of gambling on the Triwizard Tournament.

The fact that Cedric beat Harry in their Quidditch match last year (because Harry fell off his broom under the effects of the dementors) was omitted because it wasn’t mentioned in movie 3 (it’s implied, not very well, that the match was stopped after Harry fell). This gets Cedric and Harry off to a rocky start in the books.

Harry’s leaving the Dursleys’ to go to the World Cup is omitted; in the book, the Weasleys try to connect Harry’s house to the Floo network, but the Dursleys have an electric fireplace insert that blocks their path, and Arthur ends up destroying half the Dursleys’ living room in the process of extricating them.

This book was trimmed right down to the barest elements; I think it’s actually the longest in the entire series, but they condense it to just 2 and a half hours of movie. As such, there’s a LOT missing:

Dumbledore’s explanation to Harry after the events in the Ministry is cut WAY down. In the book, that’s an entire long chapter basically explaining Dumbledore’s behavior from the very first book up to the present, including why Voldemort wanted to kill a one-year-old Harry in the first place, and which also features a full-out raging teenage meltdown on Harry’s part.

The fight between Voldemort and Dumbledore is changed; the spirit is there but the spells and other events are perhaps more visually impacting.

The Department of Mysteries is much simpler in the movie, mainly centering around the Hall of Prophecy and the Ampitheatre. All the little oddities in some of the other rooms in the book, and the role they play in the fight, are omitted. The fight itself is slightly truncated as a result; there were more of the kids still able to fight when the Order shows up in the movie than the book.

The OWL testing is hinted at in the movies, but there’s much more in the books; all the practical spellcasting including another Patronus shot right at Umbridge, Harry fainting and seeing Sirius tortured during his History of Magic test, seeing Hagrid attacked during Astronomy; all omitted.

Harry has a lot more lessons with Snape than are shown, and is kicked out of them because he purposely looked at Snape’s memories in the Pensieve, not because he saw them by accident using the Shield Charm to rebound the Legilimens Charm (though that does happen).

Dumbledore projects a little more outward emotion over what Umbridge is doing to his school in the film than the book, and seems less confident in her presence.

The centaurs were portrayed much more simply in the movie, and the fact that one centaur, Bane, wanted to kill Harry and Hermione as well (for having the centaurs do their dirty work) was omitted.

Firenze’s part in Book 5 is completely omitted from the film, as is the subplot of Professor Trelawney having a taste for cooking sherry leading up to her being sacked.

Harry’s spending the Christmas holidays at Grimmauld Place is trimmed; Snape meets him there to tell him about his upcoming Occlumency lessons. In the film, St Mungo’s is skipped entirely, and so the Occlumency lessons commence immediately once it’s clear Harry is seeing into Voldemort’s mind.

In the book, Harry, Hermione and Ron visit St Mungo’s to see Arthur Weasley after the snake attack, in the process running into their old DADA teacher from book 2, Gilderoy Lockhart, who is a long-term patient there after the backfired Memory Charm. The fact Arthur was at St Mungo’s at all is glossed over in the movie.

Harry’s “guidance counselor” meeting with McGonagall, which Umbridge sits in on and in which Harry states his desire to be an Auror, is omitted completely from the film. This tidbit is first given in movie 6, more or less true to that book.

The Weasleys cause a lot more chaos in the book before they leave Hogwarts, and it’s not known at first that they’re doing it (at least not to Umbridge and the Inquisitorial Squad), until they’re caught turning a corridor into a bog.

Rita Skeeter is absent in this film; in the book, she writes a true account, exactly as Harry tells it, of the night in the graveyard at the end of book 4, which is published in the Quibbler and gets Harry in a LOT of trouble with Umbridge.

Harry’s crush for Cho is ended in a different way in the book, after a disastrous date. Cho herself doesn’t betray Dumbledore’s Army as in the movie (however unintentional on Cho’s part), but she does call Hermione out for using such a terrible curse against Cho’s friend (who DID betray them), which pretty much ends their relationship when Harry comes to Hermione’s defense.

Quidditch is completely absent from the film; it’s implied that the disbanding of school activity groups includes a blanket ban on Quidditch. In the book, Harry’s banned from playing after the first game, in which Malfoy’s taunting gets to him and the Weasley twins, provoking a fight. Ron first gets the Keeper job in Book 5 instead of movie 6, but isn’t very good at it (prompting a Slytherin song: “Weasley Is Our King”).

The assignment of prefects in the fifth year is omitted, along with the fact that Ron was chosen for Gryffindor over Harry for that honor.

Percy’s estrangement from the Weasleys from the book is largely ignored in the film; he’s the one holding Harry in Dumbledore’s office when Ms. Umbridge confronts Dumbledore about the DA, and thus he’s an enemy, but besides a sideways glance by Harry at his captor, that fact is never overtly brought to the audience’s attention.

The cleaning of the Black house (including the later-important detail of the locket being binned) was cut out completely. It is also during this chapter, I believe, where we’re originally given the name Regulus Black; Sirius’s younger brother and a key “non-character” in later books. The movies give the first mention of this name to Slughorn as a throwaway line in HBP.

The events leading up to Harry coming to Grimmauld Place are condensed a bit, especially the confrontation with the Dursleys over what happened to Dudley.

The movie was closer in spirit than 3, 4 or 5, but as the second-longest book (and the one with the most exposition) it was cut considerably as well:

Dumbledore’s funeral is omitted from the movie. Because of this, and the fact that in the movies Ginny and Harry aren’t publicly an item, Harry formally breaking up with Ginny (because he knows he’s not coming back to school and that Ginny can’t come with them) is also skipped; instead this is where the audience learns of Ron’s tacit approval of Harry and Ginny being together.

The fact that Snape was Harry’s Occlumency teacher, and that Harry was so poor at it, was ignored in the confrontation between Harry and Snape at the end of the film. The fact that Snape is the Half-Blood Prince is divulged here much as it is in the book. Harry and Hagrid putting out Hagrid’s house with their wands after it was set on fire was skipped, as was the exposition about Snape’s witch mother, Eileen Snape nee Prince.

The return of Harry and Dumbledore to Hogwarts after their field trip is simplified. In the book, they Apparate to Hogsmeade (because in the books not even Dumbledore can Apparate within the Hogwarts grounds), and Rosmerta, the bartender at the Three Broomsticks (who is under the Imperius Curse via Malfoy) urges them to get to Hogwarts as fast as possible using some brooms she gives them, leading to them arriving at the Astronomy Tower.

Harry’s feelings for Ginny are more manifest in the book, but shown in different ways. Because the celebration of Gryffindor winning the Quidditch Cup isn’t shown in the movie, Harry can’t kiss Ginny in front of Ron (and the rest of Gryffindor House) in the middle of it, which would have been funny to see.

Snape sentences Harry to detention every Saturday for the rest of the year after the Sectumsempra incident. Because of this, he can’t play the last game of the season, and has precious little time with Ginny. This omission is part of a general slowdown of the blooming relationship between Harry and Ginny.

After the incident in the prefect’s bathroom with Sectumsempra, Harry has to hide the book in a hurry, and he does it himself by going to the Room of Requirement, finding a place to stash it and then decorating a nearby bust with a wig and tiara to mark it. That tiara later becomes important.

Rufus Scrimgeour visits Harry at the Burrow over Christmas, with Percy Weasley in tow, to try to recruit him as a poster boy for the Ministry (much like Fudge did) and to try to learn from Harry where Dumbledore keeps disappearing off to; the readers learn that Umbridge still works for the Ministry, and that not much has changed in the world of magical politics after Fudge’s ouster.

In the book, whether Ron is going to live or die after being poisoned is much more in doubt; in the movie, he sits right up and cracks a joke. The next scene, where Ron says Hermione’s name in his sleep causing Lavender to break up with him, is not true to the book, though she is offended that nobody thought to tell what had happened to Ron, and Ron begins feigning sleep when she later comes to see him in hospital.

In the book, Felix Felicis doesn’t make you act as drunk or loopy as Harry was in the movie, and Slughorn isn’t stealing Tentacula leaves when Harry meets him.

Cormac McLaggen had a bigger role in the books, primarily in the omitted second Gryffindor game of the season, where McLaggen is such a bad team player and such a know-it-all about Quidditch that he ends up taking Harry out with a bludger in the second game of the season.

The fact that Katie Bell was a Chaser for Gryffindor gets one throwaway mention in the movie, and the fact that her being out for several weeks hindered Harry’s Quidditch team was completely ignored. The character of Katie as portrayed in the film was much younger; she should be a seventh-year by this point, the last remaining member of the team from books 1-3, but the casting made her look more like a fifth- or sixth-year.

Levicorpus is just another spell, not one of the HBP’s inventions. In fact, it was actually first used in film 5 (Luna uses it on a Death Eater at the Ministry), when it isn’t introduced to readers by name until book 6 when Harry uses it accidentally on Ron.

The book had a lot more memories that Dumbledore had collected, relating to what Tom Riddle was doing in his late years at Hogwarts and immediately after leaving school.

The subtext of Harry’s Auror ambitions is present but diminished in the film. We don’t see Harry’s OWL grades, but we’re told the basic information we need to know about why he hadn’t signed up for Potions, and thus how the HBP’s Potions book comes into his hands.

Bill and Fleur’s courtship, and the angst that causes the Weasleys, especially Molly, is absent from the film. We’re not even told about Bill in the films until DH part 1.

The opening chapters of Book 6 give you the same information presented in the movie, but in a very different format involving the Muggle Prime Minister.

… was pretty close on screen, compared to the last few, but that’s because they had two movies - over 4 and a half hours - to tell the story. The main things missed were:

Ginny kissing Harry at the Burrow is a bigger twist; they’ve officially broken up, but obviously not for any love lost between them. Ron breaks it up instead of George, chastises Harry for leading her on, and takes steps to make sure they’re not alone together again until the wedding party is crashed.

In the book, Scrimgeour interrupts Harry’s birthday party at the Burrow (which itself was skipped in the film), first to try to get information from Harry about Dumbledore’s death and the Order, and second, to read Dumbledore’s will (which is seen in DH Part 1). Scrimgeour in the film has a little less animosity toward HH&R than is stated in the books, but he is still rather short with them; it’s taken as part of his character.

Harry is himself in the film during the wedding; in the book, just to be safe, he’s given doses of Polyjuice Potion to look like a redheaded boy from the nearby village, and is passed off as yet another Weasley cousin.

HH&R Apparate to Tottenham Court Road in the book, but it’s given as Shaftesbury Avenue in the film, with Hermione saying she used to go to the theater with her parents. Road names in London are a bit odd, due to the age and history of the city, and the road now systematically known as A400 includes parts of both historic streets. This isn’t a major crime, as the establishing shot is the very famous Picadilly Circus where Shaftesbury starts, which is a few blocks southwest of the major theaters on Shaftesbury. Tottenham Court Road proper is only a few blocks north of there following Charing Cross (also part of A400).

The discussion with Elphias and Ron’s great aunt is longer. In fact, the entire tarnishing of Dumbledore’s shining reputation and the introduction of his backstory was a subtext almost completely removed from the films, with only the barest elements given to the audience where it’s absolutely essential. This results in a reduction in Aberforth’s part late in the second film, and of Dumbledore’s at “King’s Cross”.

Viktor Krum is in attendance at the wedding in the book, having been invited by Fleur. Harry makes the leap that Gregorovitch is the wandmaker that Voldemort is looking for after remembering that Krum’s wand was made by Gregorovitch (from WAY back in Book 4 during the weighing of the wands).

Harry, Hermione and Ron spend more time at Grimmauld Place planning the raid on the Ministry, and Kreacher actually warms to them after the confrontation in the kitchen over the locket and Mundungus Fletcher, before HH&R are forced never to return after escaping the Ministry.

Tonks and Lupin being engaged/married is brushed over in the films. In the book, it’s a much bigger subtext; Lupin shows up at Grimmauld Place while HH&R are planning their infiltration of the Ministry, and we learn he wants to leave Tonks because she’s pregnant, and he’s afraid what the child will be (he’s a werewolf after all). Then, while HH&R recuperate at Shell Cottage later, Lupin stops by and gives the news that it’s a boy, and asks Harry to be godfather. All of this was removed from the film plot, which I think was a mistake; it removes some of the power of the events of the fight.

Phineas Nigellus is absent from the film (and pretty much all others; he has a bit part in book 5 as well). This forces the film to be very vague about how Snape could have known where they were to get the sword to them; there are memory images in the Pensieve scene but no details.

In the book, the spark for Ron leaving Harry and Hermione is an omitted conversation between Tonks’ father and two ex-Gringotts goblins, overheard by Harry, Ron and Hermione while camping, regarding what’s been happening in the outside world (particularly to Ginny, the last Weasley at Hogwarts).

Harry had to make a conscious decision in the book, choosing Horcruxes over Hallows, by choosing to stay at Shell Cottage to get the information he needed from Griphook and Ollivander, instead of acting on the knowledge he has through his connection to Voldemort regarding where the Elder Wand is and that Voldemort also knows. In the film, there was never a chance that Harry could have gotten to the Elder Wand first; HH&R were still recovering from shock and injury from the events at Malfoy Manor.

At Gringott’s, Harry uses a few more Imperio charms in the book, and also Confunds a few guards at the door.

In the Lestrange vault at Gringott’s in the book, the treasure has not only been guarded with Geminio, but also the Flagrante curse, which causes the copies to be burning hot to the touch, making HH&R’s entrapment in the burgeoning treasure that much more painful.

Griphook’s role in the movie is close to the book; in the book we’re not told he’s killed at Gringott’s, which I think is a good foreshadowing, as Neville getting the Gryffindor sword from the Sorting Hat at the very end could be confusing to a reader who knows it was last seen in Griphook’s hands.

The Carrows are omitted from the movies as speaking roles, through they’re mentioned in the right place in the film and it’s inferred the two of them are the ones flanking Snape in the Great Hall; in the books they’ve taken over Muggle Studies and the class formerly known as Defense Against the Dark Arts, and Harry uses Crucio on one of them in Ravenclaw Tower to protect McGonagall (for the first time seeing what Bellatrix meant: “you have to mean an Unforgivable Curse”). McGonagall also uses an Unforgivable (Imperio) on the other Carrow in the same scene of the book, showing that Harry’s uses of them in several situations, while extreme, are not unique among the good guys and so not necessarily “unforgivable”.

In the book, after the Grey Lady (Helena Ravenclaw) confirms that Tom found her mother’s diadem and brought it back to Hogwarts as a Horcrux, Harry remembers putting a tiara on the bust he used to mark the location of the Half-Blood Prince’s potions book; that’s what allows him to find the tiara again. The film simplifies this by inferring Harry is attuned to the Horcruxes and can sense their presence.

Most of the battle plays out slightly differently; the whole scene with Neville on the bridge isn’t in the book, and there aren’t as many of the past characters shown in the movie as the book has. Colin, Wood, Lavender, Cho, Percy, Bill, Charlie, Alicia Spinnett, Katie Bell, even Grawp and Kreacher are all mentioned, and Percy gets a reconciliation with his parents before the fight. Different people are in different places, and different things happen, but the spirit is there. I did want to see Professor McGonagall directing a herd of animated classroom desks down the hall with a most un-McGonagall-esque “CHAAAAARGE!”; her use of Transfiguration in the film was limited to one spell animating the suits of armor (more like statues in the film).

More Pensieve memories in the book than the film, of course, though they showed more than I thought they would in the movie. The film simplified how Petunia became so virulently anti-magic, why Lily falls away from Snape at school, and how Harry got the Sword of Gryffindor. There’s also a minor plot point in the book where we find out who ransacked Grimmauld Place (besides Mundungus) and why half of a letter from Lily to Sirius was missing, as well as half of a picture.

The scene at “King’s Cross” in Harry’s mind is truncated, mostly omitting the parts where Dumbledore fills in the remaining blank spots of his backstory; this entire subtext was largely absent from both parts of DH on screen.

The killing of Nagini is much more straightforward in the book; Neville breaks a “Full Body-Bind” curse Voldemort puts on him, pulls the Sorting Hat off his head (Voldemort put the hat on him and set it ablaze while Neville was paralyzed), draws the sword from it and chops the snake’s head off before the battle even starts up again. The film draws it out to a last-minute thing.

In the book, the centaurs and other creatures from the Hogwarts grounds join in once the battle starts up again. Their role is omitted, though we do see the spiders attacking earlier in the battle, having been driven out of the Forbidden Forest by Voldemort’s forces.

In the film, the audience learns why the last standoff between Voldemort and Harry plays out the way it does after the fact, instead of while the two are staring each other down as in the book (though Harry hints to Voldemort that he knows why the Elder Wand won’t work for him). I actually think that works better for the movie, but in the book, the slow-burn realization that Voldemort has lost this final duel before it even starts is a nice touch.

In the book, Harry is able to use the Elder Wand to repair his old holly-and-phoenix-feather wand (which was snapped in Part 1). In the movie, it’s implied he simply continues to use the one he stole from Draco.

Instead of snapping the Elder Wand outright, in the book Harry simply chooses to place it back with Dumbledore in the tomb and let nature take its course. If Harry dies a natural death, the wand’s power will be broken. Harry’s rejection of the Elder Wand’s power is more forceful in the film, and also more in keeping with his ambitions to become an Auror (a job which, as is illustrated many times, doesn’t lend itself to a peaceful death in bed at a hundred and twenty years old).