I'm really looking forward to see the new quests changes.
I really loved how the quests where setup to work together in WOTLK and in TBC, and i am really looking forward to see how they incorporate that quest design into the new world + all the phasing!
I'm really looking forward to level trough the new content due to this.
I'm also looking forward to trying the new rune mechanics for Death Knights to see how it works.
And i'm really, really looking forward to venture into Skywall! That is gotta be the most epic looking instance i have ever seen in this game!