I like UaW because of its free play features. I don't like campaigns, I like playing on LAN. Yes, I did forget supreme commander.
TF2 is why I want Orange Box (but I also want to play Ep.2 and Portal). I love the dynamics of TF2, simple and elegant.
COD4 is awesome.
I am not sure about Left 4 Dead yet. Playing the soldiers sounds like fun but I am not sure if it would have lasting value. I have no interest in playing as a zombie, but if I got the game I would give it a shot.
I would cite regulation, but I know you will simply ignore it.