L4D is one of the better co-op games. I don't know how people manage to rack up thousands of hours on it, but I've put in over 100* and that is quite a bit since FPS games are one of my least favorite genres. I think the last FPS game I really got into was Goldeneye 64? Halo/CoD etc. never caught my interest.
*prolly would be more but only got the game in 2012, well after the initial release.