I wish TF2 had a co-op campaign mode rather than just being a pointless arena/deathmatch-type game. The characters and style it has are fun.
Well, there's the "Mann VS Machine" horde mode if you want co-op action. Deathmatch isn't the first game type that comes to mind when I think of TF2; the game's based around objective-based modes such as payload, king of the hill, capture the flag, attack/defend, and capture points.