Can you imagine Left 4 Dead without co-op? At this point, it would be like Street Fighter without a versus mode, or Keith Vaz without the the complete and total ignorance. Strangely enough, however, this co-op game began life without co-op even being a factor.

"Originally we had what would be considered Versus mode now: that was the main game. There was no separate Co-op," explains Valve's Chet Faliszek. "What we found was that we wanted to make the highs and lows of the co-op experience more distinct. [We had] this problem that if the infected pound on the survivors then their reward for kicking their butt would be, 'Hey you don’t get to spawn for 60 seconds because we want to have this really big lull.' Well, that’s not a good reward.

"At some point we separated the two… The splitting of those in the process is probably the biggest change we’ve made. It’s really helped define the game."

Funny the way things turn out, isn't it?