The main thing that got me excited, and is probably the place I'll treasure the most in the upcoming expansion, is the Skywall. I've been waiting a long time for a change of scenery from the old lava/cave/mostly-linear design of old dungeons (and most new ones) that always seemed to be in-doors or simply felt constricting.
Being a big fan of any and all things that are flight/storm/air, I can't wait to try out the, to my knowledge, only dungeon and raid who's main theme is elemental air.
The second thing I'm excited about is the great ammount of lore we get to experience in the goblin/worgen starting zones, later in the Twilight's Hammer infested zones, and what becomes of the Earth-Warder.