Hrm tough to pick just one.
Mage - obviously the 90 talents are terrible and could use a redesign. I'm also of the same opinion as others that have said it would be nice to play a spec that doesn't revolve around dotting it all up. I originally rolled a mage for direct damage abilities, not DoTs.
Warrior - I -despise- Colossus Smash for PVE. I quit raiding as a Warrior because of how much I hated it. The entire combat system is now all about pooling procs and rage to unleash everytime CSmash is up. It's boring and annoying. Fury was supposed to be the random spec hitting whatever pops up, but that isn't what happens now. I'd also love to see Arms as good as Fury for PVE DPS for once. Or even *gasp* ahead of it for awhile.
DK - Agreed the level 90 talents need a change.
Rogue - Oh my goodness! Could Assassination be any more boring? I don't think it could be. Do you guys remember when rogues were known for their big attacks? Assassination used to be all about big quick burst. Then it slowly turned into the majority of the damage coming from autoattacks. I miss the old feel of rogue.
Healers in general - I miss it when every healer had their own niche. Paladins were pro tank healers. Druids HoTTed it up. Disc good for tank healing while Holy is all about AoE healing etc etc. Every healing spec is too similar for my tastes.