PC has always bred a more competitive than cooperative gaming scene... but with Left4Dead I imagine that the culture will shift depending on its success, I can't remember any title on the PC in the past that was sold upon a coop centric experience - if the popularity of it is as high as expected then im sure a lot more games of its breed will begin to be announced.
I agree. I wasn't too fussed about this when I heard it was an online game, until I saw just how much it's like a multi-player, single player game, now I've ordered it. Will apparently feature split screen too- I hope that comes to PC as well as Xbox, beacuse it would be fun to have 2 players at the PC and 2 from the interent all cooperating.