How fast does a T rex run? 20 years ago, the technicians at George Lucas's effects house, Industrial Light and Magic, laboring to finish Jurassic Park in time for its June 11 release date, made a decision whose effects would reverberate for decades to come. "We had a zillion arguments about it," said animator Steve Williams. Some argued, based on the animals' estimated mass, that it ran slower than a jeep, the only problem being that a jeep was precisely what it was required to chase in Spielberg's film. Others argued that it ran more like a lion: never unless it had to, and if it ran, only for a very short period of time, moving very fast. "Using that logic," said Williams, "I had to throw physics out the window and create a T rex that moved at 60mph, even thought its hollow bones would have busted if it ran that fast."

That decision – cheating mass to achieve the desired velocity – set the pattern for the bendy new laws of physics that were about to unfurl. The 80s had been about nothing if not mass, when 'roid-head heroes came on like one-man biological armies, wiping out whole buildings, neighbourhoods, villages, with one clench of oily pectoral. "He fills the space, and you have to go with that," said James Cameron of Arnold Schwarzenegger when casting The Terminator, originally a lithe assassin with a buzz cut, upturned trench-coat, capable of disappearing into a crowd.

What he got was 220 pounds of Austrian bodybuilder, who could no more disappear into a crowd than he could perform a pas de deux. As the 80s boomed, so too did the biceps of its movie heroes. "I always believed the mind is the best weapon", insisted John Rambo, before strapping some beefy rocket launchers to his forearms, in case his mind wandered.

Schwarzenegger's character in the 1993 film was the last action hero, and, for a while anyway, it was. Photograph: Snap/Rex Features

That all changed in 1993. "Lizard eats Arnie's lunch," ran Variety's headline, after Jurassic Park smushed Arnie's appropriately titled Last Action Hero at the box office, ushering in a brave new world of computer-generated effects in which the bulk of Arnie, Sly, Bruce and the gang was suddenly a drag – un-aerodynamic. As The New Yorker's David Denby wrote in his review of X-Men United (2003):

"Gravity has given up its remorseless pull; one person's flesh can turn into another's, or melt, of become waxy, claylike, or metallic; the ground is not so much terra firma as a launching pad for the true cinematic space, the air, where bodies zoom like projectiles and actual projectiles (bullets say) sometimes move slowly enough to be inspected by the naked eye. Roll over Newton, computer imagery has altered the integrity of time and space."

This brave new post-Newtonian universe would belong instead to swift, svelte, low-cal metrosexuals like Keanu Reeves, Matt Damon, Leonardo diCaprio, Joseph Gordon-Leavitt and Andrew Garfield – buff but not ripped, able to cling to a window ledge by their fingertips, or run upside down a corridor, or wrap their tonsils around the gobblydegook of a script like Inception, whose exposition levels alone rendered it a no-fly zone to monosyllabic grunters like Sly and Arnie, whose eloquence at best stretched to a gruff "screw you" as he plunged a power drill into a man's chest. Jurassic Park put paid to all that – it put machismo on the extinction list.

The rerelease of Spielberg's frightener, newly refurbished in 3D, offers plenty of opportunity to ponder these and other important matters, such as: why isn't it more frightening? Why don't those night-time binoculars ever get used? Why doesn't the film have an ending? And: what's with the first 20 minutes? Spielberg's beginnings are usually beauts, too. Jaws begins with such fluently mounting dread that we are 30 minutes in before we realize that nobody has said the word "shark". The same with Close Encounters, which goes 40 minutes without anyone mentioning UFOs.

In Jurassic Park, it is five minutes before people start in with the dino chat, and they never let up: we get dino history and dino DNA, dino facts and figures, ticker-tape reams of dino data, and then – at the 25-minute mark – our first dinosaur, munching leaves in a field. The cast dutifully adopt their shock-and-awe expressions, although how it crept up on them, given that it is as large as a house and stood in a field so wide you half expect to see your dad practicing his golf-swing, is anyone's guess. David Koepp's script had them walking in a forest, coming across a tree trunk, then three more, the trunks turning out to be legs – a much more Spielbergian alternative to the version Spielberg gives us.

In other words, this is his you-ain't-seen-nothing-yet moment, and he fluffs it. Jurassic Park may have re-angled the trajectory of Hollywood as thoroughly as did The Jazz Singer in 1927, but like The Jazz Singer, it is not a particularly good movie. It's one of the big secrets of film history, in fact, that cinema's landmark films often turn out to be lousy, frequently for the same reason that they are landmark films: too busy funneling this historic head-wind, or showing off that game-changing technological development, to keep their balance on the beam.

Spielberg's heart and mind were elsewhere – on Schindler's List to be precise, which he shot almost concurrently, as editing on Jurassic Park wound down. His sensibility had bifurcated. The 28-year-old who made Jaws with such disinhibited glee now worried about the kids in the audience, those raptors in the front row. "Frankly I don't think that Jaws would do as well today as it did in 1975," Spielberg would say when Jaws was rereleased a few years later: "because people would not wait so long to see the shark. Or they'd say, there, too much time between the first attack and the second attack, which is too bad, we now have an audience that doesn't have patience with us. They've been taught, by people like me, to be impatient with people like me."

Jurassic Park is a product of that impatience, and whisked it up some more. By 1992, the entertainment industry had become America's second largest export after aerospace, generating some $3.7bn in revenue, and in 1993, thanks to Jurassic Park, overseas box office out-stripped domestic for the first time in Hollywood's history – a crucial tip of the see-saw, for that global market would increasingly be calling the shots. "The world is listening," declared the motto of George Lucas's THX Dolby system. In truth, it was the world that was now speaking. It was Hollywood's turn to listen.

• Jurassic Park 3D is released worldwide on Friday, April 5.