Limited and banal ability sets.
Why? Because of the need to keep tight raid balance. If any one Class/Job gets a slight edge, the player base explodes in self righteous fury.

Any time the developer wants to add anything, they have to be ultra careful with how it affects balance. Want to add something because you think is cool or interesting but will impact raid balance? Forget it.
Banal gear
Same problem as with ability sets. "How will it affect raid balance?" becomes the overriding concern instead of "Is this interesting or fun for the player?".

Want gear with Poison resist? Nope. If there is a raid boss that uses a lot of poison, acquiring said gear becomes "mandatory" and players explode with self righteous fury that the game dare make them do something that they might not be interested in.
Banal items
Same story. If raid balance says, no. It's not happening.
In general, raid balance constrains the game's creative direction.

Want to introduce some new cool lore but it might clash with existing (balanced) player abilities? You either don't do it or you break suspension of disbelief by introducing a contradiction.

WoW is the logical conclusion of this approach. Raider's needs run the game. Everything is sacrificed to their benefit. Lore is shot. The world is boring.

People complain that 3.0 is "more of 2.0". Well of course it is. Any kind of change will have to be made from the perspective of maintaining the precious raid balance. The developer's hands are tied - with rope of their own making but tied nonetheless. They can't just introduce any old cool idea they get willy nilly.

Is there a way out of this dilemma?
IMHO just make raids Job-only and keep Jobs (and their equipment access) limited, boring but balanced - i.e. like now.

With Classes, go wild. Introduce all sort of strange cross-class skills usable by classes only. Have all sorts of weird gear with odd effects for Classes.
You can probably sense by now I'm not big on raiding. I play the game for atmosphere, to explore a fantasy world, kill monsters is just one of many "activities" to me.