With L4D, we want to give you something that's not just totally mindless to play because you and your buddies have to work together. We call it co-op for lack of a better term, but I think you're going to see a lot of multiplayer games go in this direction.

We've already seen games like World of Warcraft and the MMO scene doing this - you have parties, you stick together, you work together, and obviously it's really popular, so why shouldn't action games be going in that direction as well?