Personaly, I'm really waiting for the new race/class combos.
Always wanted a human hunter!
Also with the redoing of the old world, along with the phasing with quest. Finaly actually see my efforts in quests show up! Can't tell you how much I hated seeing nothing change after doing all these helpful things for a quest hub.
I'm also very happy to see all the class overhauls with mechanics. Everything but rogues and warriors (before death knights) having mana, was rather boring. I think adding the little different class mechanics is a nice way of not having everything feel the same, and making playing each class unique.
But yeah, thats the things I'm most excited about with Cataclysm. In general, I really can't wait for this expansion to come out, it's definetly not just another "We get to kill this guy and get a new zone". It's actually expanding on the things from the old game, rather than just making you forget about it.