For me? NPC's, by far. Just like the ambient sound-effects and the meta-events did, NPC's will bring a whole new level of life to the game. I want to be walking down the street, to find a barricaded house and a bunch of dead zombies outside. What will I choose to do next? Will I approach the house in hopes to find a group to survive with? Will they attack me? What if they're friendly but one of the survivors is infected? What if I wake up one morning and all my supplies have been taken? That's the beauty that NPC's offer, and I can't fucking wait for it.