I'm not sure how many other people have said this, but I'm most excited about the crafting system changes!
Anyone who knows me in-game (Adelpha; Kilrogg server) will tell you that I am easily the most fanatical blacksmith on the server. I'm kept pretty busy, and I make decent money at it. My greatest excitement about Cataclysm can be distilled into a single word: REFORGING.
In the first place, this is going to keep me extremely busy, which means even greater income from my smithing. But it also excites me as a player, since I'm a ret pally. Lots of the dps plate that drops has armor pen, which is awesome for warriors and DKs, but completely useless for us retadins. The ability to turn that armor pen into crit--which I can really, really use--makes me very, very happy. It will also allow Blizz greater flexibility in designing drops, so that they don't have to make plate specifically for retadins, for example.
Also, I seem to recall something about being able to combine two items such that you get an item with A's appearance and B's stats and so forth. Great for deception in PVP, but also just great for those of us who are vain about what our toons wear.
Another change that will hopefully get new blood into all the professions is that making items that are orange for your character may yield more than one skill point per craft. Painful leveling will become less so. This can only mean more crafters in the long-term, which is good for everyone.
In closing, aside from all the changes I'm simply looking forward to the race: the race to 85, the race to 500 BS skill (or whatever the new cap is), and the race to find all the drops and farm all the rep to get the high-end recipies so I can make them for my fellow power-gamers.
HORDE RULES! ^^