At this point, WoTLK is a letdown. They did some things well, including interesting zones and improved quests. The 10 man and 25 man versions of raids allow a smoother, faster progression. Achievements, like the special dungeon/raid ones, are nice. Otherwise, many things are a big failure.
PvP is majorly imbalanced. This isn't a resil issue, the game already favored melee and wotlk only makes it worse. The situation is so bad, I sort of wish they'd just nix it all and issue a public acknowledgement of WoW being a PVE game and to get PvP elsewhere. I just don't think Blizz has the people to do PvP right. They don't have the mentality for it. Admit it and move on. Don't make me play your broken arena so I don't get gimped in the world.
So many things brought up during the Beta as problems were ignored, went live, and are just being acknowledged now, months later.
Gear graphics are probably the worst I've ever seen. It just looks like a lazily done job. If the time they saved on not making interesting gear was spent elsewhere, it sure doesn't show.
There just isn't that much content. I've only not done 25 Malygos, and from what I hear, its actually easier than the 10 man version.
Some classes seemed to had been ignored during wotlk development. This again relates to Beta feedback being ignored.
Wintergrasp was a great attempt at bringing back world pvp that is hindered by being way too buggy to ever have been released like it is.
On top of this, Blizzard PR feedback on these problems have been snotty and abusive. The normal response pattern is: 1. Deny the issue exists. 2. Threaten users who continue to bring it up 3. Say "we'll look into it" 4. Wait a month. 5. Either go back to 1 or begin a long, slow "fix"