I love NPCs like this -- the NPCs that do curious things like punt you all the way across Azeroth, or murder unsuspecting lowbies that don't understand that right-clicking a yellow conned NPC will result in a swift punch to the face. The NPCs with stories, like the saga of Grifta from Burning Crusade, and the continuing adventures of Investigator Asric and Peacekeeper Jadaar.
While they serve no real purpose, they're always there to entertain, buff, or amuse. There's always an NPC or two that never fail to bring a smile to my face, and that I look forward to meeting on whatever alt I happen to be playing. Who's your favorite NPC? What's your favorite little thing in the game?