There're so many cool things in Cata, i relly don't know what's going to be the coolest, but excited about the changes in how u play the game. The new talents are really awesome, and even though the talents were already packed with a ton of cool stoff, they came up with more talents, that's really going to make leveling and raiding much more fun. I also think the way it's going to work with the swaping between geting a new spell and getting a talent is great. We have so many talents at the moment that's just fillers, so it wont really matter that much, that we only get a point every second level.
I think the entire gameplay changes along with the world changes and the addition of the new races and class/reaces combos, are really going to make WoW even better. Right now leveling is just filler stoff, but as i see it, the leveling experience will be just as great as raiding or PvP, and i think that's what i'm most excited about. To be able to have fun while leveling. Alternate between questing and dungeons, and being able to play a new race, or playing as a class/races combo i've never tried before.
All in all i think Cata will give me what WotLK couldn't: Something to do while not raiding. :)