The thing I am most excited about in Cataclysm are the new dungeons. I have watched videos and read reviews of the first three dungeons that have been released in the beta so far and it really seems like Blizzard is making a strong effort to make the dungeons a lot harder and more exciting like they were in Burning Crusade and Vanilla. I'm really looking forward to heroic dungeons being truly heroic again.
I am also really looking forward to leveling alts (both Worgen and otherwise) through the new "old world."