To me, one of the cool aspects that hasn’t been talked about in Left 4 Dead is the “Director”, an AI system that creates encounters for you as the game goes, meaning no 2 single-player (or co-op) playthroughs might be the same.
In versus mode, this changes. You play as a zombie, which makes you the “Director”, designing the campaign experience for your friends, in a sense. You can even call other zombies to your aid to battle your human foes. A new video interview has appeared on the webz, highlighting some of these features.