I find it rather hard to pick just one thing that I'm excited about, as there's just so many things being added. If I did have to pick one thing in specific though, it would be the change in healing mechanics. I agree with Blizzard about current healing not requiring much skill, and being too spammy. Having healers actually care about how they use their mana more, along with using the right spell for the situation, and having groups as a whole care about the damage they're taking again, is an excellent and needed change, that I can't help but love.
Although, as much as I love that change, it only slightly beats out the other amazing changes, such as the 31-Point Talent Trees that are shaping up to be very interesting, or the ability to fly in Azeroth. There were many times leveling up that I thought to myself: Gee, it would sure be awfully nice if I could fly, imagine what a better view I'd get of the beautiful world. I'm also excited about the raid change, that 10 and 25 man raids give out the same level of gear. I disliked doing both each week, as I found it far too time consuming, not to mention boring to kill the same boss for the second time that week. Then there's the Worgen, I mean how can't one love British werewolves?
All in all, I'm excited about the expansion as a whole, it's going to be quite amazing, and I can't wait to experience the cataclysm first hand.