Not to get this argument going again, but I just finished Painkiller the other week and can't understand why People Can Fly couldn't have included coop. I mean, the gameplay is very, very similar to that of Serious Sam. Yet I read some comment by one of the devs that they'd have needed another year of development time to include coop. Why? What is there to do? Spawn in some more enemies? The levels would all easily accommodate two players (okay, the asylum might be a little cramped in spots, but so what?), there are no plot points to worry about if you cut the few between-level cutscenes, no triggered events other than spawning enemies, etc.
Painkiller would be a natural for coop. And without coop, there's little point in replaying the game, unless you're really into the whole tarot card thing (which I'm not). Great game in many ways, but it's only half-there without coop.