With all respect I must disagree with the idea that L4D's emphasis on co-op is a good thing, the thing is that it puts too much emphasis on this by (more or less) killing anyone who deviates from the group (even if it's for a good reason such as finding ammo, health kits or going back to revive fallen team mates, unless you have someone holding your hand at all times you will probably die) which is unreasonable at the best of times and just plain stupid at the worst.
I shouldn't be forced to work with my team mates (indirectly or otherwise). I should want to help them and commit acts of altruism in-game because I feel inclined to, not because my hand's been forced in a 'help or die, motherf**ker!*' scenario.
This has also led to a large number of players (largely over XBL, I'm not sure about the PC version) who will boot you out of the game for something as simple as going into the next room without forming a crocadile with the rest of the team first.
I understand that Valve didn't just want lone badasses to just run and gun through the entire level without caution (which would make having a team of four survivors pointless) but I think they took the 'dependance on each other' too far, which in turn detracts from the fun and enjoyment of the experience.
