As soon as the expansion hits, I'll be rolling a goblin warlock, and I imagine it's gonna be a lot of fun with the updated zones, the new quests, and not to mention the talent overhauls making low-level a lot more interesting. Also I hope to see the quests and their story tie the zones more together (and preferably long questlines!), much like how it was in Northrend.
I really think the leveling experience is what I'll enjoy the most during this expansion.
Sure, I'll probably want to see the raid content and such at level 85, but raids is the thing I care the least about. In fact, I'm much more excited to see the 5-man dungeons at level 85 and whether Blizzard have decided to add a dungeon set like the ones in vanilla and TBC.