While I'm excited about all kinds of different things with this expansion (the new races, the new class combinations), I'd have to say the one thing I'm excited about most is starting over.
Never before in WoW have we been faced with such exciting choices of starting anew in Azeroth. Along with the new races, we'll have completely revamped zones, new quests, and smoother quest transitions (from area to area). The new talent trees, and the new abilities we gain at level 10 only add more to it!
I enjoy my level 80 warrior immensely. But I will gladly recreate him (as a worgen or goblin) just to experience an entire NEW experience of leveling a character, as we haven't seen in the past 5+ years in the World of Warcraft!