Personally, I'm looking forward to all the new engineering recipes that will be showing up, especially if some of the ones already datamined are any indication of what to expect, as well as seeing the changes to my class of choice, hunter.
Besides that, since I play alliance, I'd probably have to say that I would also be looking forward to maybe playing a worgen, but more than that would have to be simply the lore progression from what was already established to something that is more or less brand new to Warcraft. I'm already itching to start playing again, and a chance at the beta would make it that much sooner.
I'd have posted this sooner, but I forgot my login and had to get it through email again... oops ^_^;
*fixed* posted on wrong section