Currently (late 2016) there are only a few really impactful stats and if an item doesn't have enough of at least one of them, it's probably a bad item. (DISCLAIMER: Make sure you have enough resolve for the content you're doing.)

So first let's talk about high-value stats since they're the most important.

Crit bonus, potency and ability mod:

These are in a league of their own in terms of character performance impact. Their relative importance varies a bit by class but they are generally the most sought after stats by literally everyone who knows what they're doing. They determine how much your spells, combat arts, heals, wards, taunts, etc hit for. The more you have of these, the bigger your numbers will be.

Of course tanks have to value survivability, and healers who don't care about dps can cap these out in the healthpool/HoT tick/Reactive trigger relationship. But for the most part these are where the performance comes from. Tanks and mages may sacrifice some of these for survivability and doublecast, but they're still looking for as much of these as they can get.

Next let's take a look situationally high-value stats:

Health, block, mitigation, doublecast.

For tanks (and in the case of doublecast, crusaders, caster-healers and mages) these can be as high or higher value than pot/cb/ab mod. However, most of the good tanks will generally not give up, say, a 900 potency/zero defensive stats wrist for a wrist with 16 block and 10 mit, which by current standards is an exceptional set of defensive stats for a wrist. The point here is that classes that need these stats are going to flock to the items that provide them, in addition to, the most cb/pot/ab mod they can get. Items with only these stats and little or no pot/cb/ab mod are generally considered useless. The amount of just these situationally high-value stats an item would have to provide in lieu of the high-value stats would have to be very great in order to entice good players into wearing it.

Most of the determination of whether an item is good or bad comes from the amounts of high-value and/or situationally high-value stats it has. Put simply, most items that don't have a big chunk of crit bonus and/or potency are going to be seen as junk in a post-GU100 Norrath.

Next, what I call "chore" stats. (Because keeping them reforged to the right values is an unglamorous CHORE that you have to keep up with every time you get a new piece of gear) These are stats that players want/need to have at a required level in order to not somehow be nerfing their performance:

Reuse Speed, Casting Speed, Recovery Speed, Hate Mod, Strikethrough, Accuracy, AE Auto and Haste.

Everyone needs 100 of the first three. Tanks need their hate mod and Strikethrough, and most people want 20 accuracy for hit rates, and 100 ae auto and 200 haste to maximize proc rates. They aren't fun or glamorous stats, but without them the player is nerfing their potential. They aren't a huge balance factor as they are mostly handled by reforging. Amounts of these stats in excess of what I listed are mostly wasted and have little performance impact.

Note:100% Recovery speed reduces the time till you can cast the next spell from 0.5 seconds down to 0.25 seconds. It is more meaning full now since it's less important to time autoattacks

Note:Everthing above 100% recovery speed or reuse speed gives no benefit.

Note:Everthing above 100% casting speed converts to Spell double cast chance, which will be not benefit a class that has only combat arts (evade and taunts are listed comabt arts but can be dauble casted

And finally low value stats:

Flurry, DPS, Multi-attack, Weapon Damage Bonus, Spell Weapon Damage Bonus. These stats have value mainly in direct ratio to the amount of ability mod they can be reforged into, along with some value from prestige conversions and spells like battle cry that convert them to more relevant stats like potency. An item with 300 weapon damage bonus and 100 flurry will increase a good scout's DPS less than an item with 500 potency and so we see that, when even absurd amounts of certain stats have little positive impact, you know they are garbage stats. Therefore items which mostly have these stats will be seen as garbage items, unless they also have an acceptable amount of high value stats. Items like these are the ones I hear people saying "why are they wasting dev time making these?" "why are they wasting database space on these?".

Summary: Every class should stack Crit Bonus, Potency and Ability Mod. If you're a tank you may have to trade some for health, mitigation and block. And if you're a class that benefits from doublecast you probably want to get 100 doublecast. After that make sure you have the right amounts of the "chore" stats, and try to reforge such that any low value stats left over line up with your prestige conversions or buffs like battle cry that turn them into something useful.