Three things I'm most looking forward to:
1. The new lore, particularly surround Deathwing, Mount Hyjal, and Vashj'ir. New stuff is always cool, and these carry on storylines dating to the War of the Ancients, and even the return of our friends from ZA. That, and there's rumors of a certain demigod (or...5) returning. Sounds delicious to me.
2. The "new" leveling experience from Lv. 1 to 58. After leveling 4 toons thru the current Azeroth, the change is welcome. I'm actually saving at least two characters on my main server for the new experience, since it's fresh and hopefully less of a boring grid with ridiculous quest flows (ask ANYONE who did the "It's a Secret to Everybody..." chain about going Un'Goro => Winterspring => Tanaris).
3. The changes to class mechanics, especially hunters (my main is one). I've long had gripe with Aspect of the Viper being a huge dps loss, to the point of learning to "Stance Dance" (of sorts) between Viper and Dragonhawk for high-damage shots. Now that viper is gone, though, I'm extremely eager to get my hands into Cataclysm early so I can get my head around a completely new resource for my class. It's gonna be a new learning curve, and while I imagine it as an energy/rage hybrid, managing it effectively is a different animal.