It seems the co-op mode of Left 4 Dead wasn’t initially planned as part of the game according to Chet Faliszek, interviewed on the Major Nelson podcast. When development started it seems all that was intended was the versus mode, but complications arose when the extensive playtesting began, mainly due to the pacing the director wanted.
More beneath the cut.
Here’s Chet’s words in full on the matter:
Originally we had what would be considered Versus mode now: that was the main game. There was no separate Co-op.
What we found was that we wanted to make the highs and lows of the co-op experience more distinct. [There was] 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.
Of course, what with all the reports coming in stating how much stronger the versus side of the game is, this isn’t really surprising. What is, however, is the fact that the co-op is so strong on it’s own. With L4D coming out tomorrow, it just makes me salivate a little thinking about how incredibly powerful this game is going to be, and just how much of my time it’s going to leech over the next year. If it’s anything as good as TF2, it’s really going to be something.