I am most excited about all the new lore and seeing what will happen to the setting.
I'm excited to see all the quest changes, all the new quests, and what happened to some of my favorite characters and storylines. Sure, the new class mechanics and talent trees are interesting, and sure, it will be fun to play as a worgen, but the primary reason I want to see a new expansion, especially on this scale, is how much it changes the story.
Burning Crusade added new stuff, had some interesting plotlines, outland and flying were neat. Wrath of the Lich King added another continent to azeroth and it had a lot of interesting things, too. But while both of these were interesting and did advance the plot, the leveling experience and most of the storyline other than the main parts of the expansions was kind of left in the dust. The 1-60 content has seen very little changes for the most part over the past two expansions, but all the changes in cataclysm promise to change that. Make the leveling experience more interesting, advance old plots, and revamp all the old zones.
As an altoholic and simply because I love the lore, I'm very excited to explore it all in Cataclsym.