I've always been a co-operative game player, whether its playing the medic on squad based shooters and helping out my squadmates or whether it was in my youth teaming up with friends to crush the opposition in the Age of Empires series... but the co-operative king has to be L4D! Never before has a game forced people to work together just to make any progress at all! Because of this I find that L4D is an acquired taste. Most of my friends are too competitive and hate the idea of working together, which saddens me!  
I'm currently away from my beloved L4D as I'm on holiday but last time I played online there weren't too many games too join but with The Passing just around the corner I hope it strengthens or rekindles peoples interest in such a fantastic game... I just hope there aren't too many lone run and gunners spoiling my games when I eventually get back online.