I very much agree with his initial statement about co-op being that it's one person eating up half your health/ammo/etc (it was a laugh to my friend and I since that's what he does in most games).
Left 4 Dead is probably the first game to do a lot of things differently and better, the co-op as mentioned actually has a level of depth to it where you have to rely on your allies or you're boned, and certainly Zombie survival considering the bajillion mods for games which, most are half way decent, but this has been the best I've seen.
And it's true, you do see people pray to the Omnipotent AI director for health kits, guns and of course no team destroying tanks.