There's a ton of fantastic new NPCs in Mists, and it's going to be really hard to pick a favorite. However, the one that I HAD to feature first was, without question, Uncle Gus in the Shrine of Two Moons (Horde city).
He's just one of those random NPCs who are scattered around towns for ambiance, doesn't offer any quests or anything, but you can talk to him (this is one of my favorite new features in Mists, it really adds to the immersion).
What does he say when you talk to him?
I love this guy.