I was playing a lot of Castle-Crashers Co-op lately and I thought about why I like it. They didn't put in many teamwork moments like Gears of War has, the ones where you have to both abuse your A button. There was actually, very little in terms of Co-Op mechanics.
But it does promote asocial behaviour to some extend, it is encouraged to steal coins or the PVP "fight for the princess' kiss" after each time you rescued a princess.
Comparing it to Gears of War or Left 4 Dead, you can also screw somebody over, but not without getting the feeling that you are playing against the rules of the game.
By introducing a bit of Anarchy, making "bad" behaviour acceptable to some extent may be more fun than not, you really have to trust your fellow gamers, there is no invisible game master holding them back, or magic bullets that do not hurt you. Maybe it ultimately serves to make Co-Op more interesting, more diverse.
This works very well in the cartoonish sarcastic world Castle Crashers is set in, maybe it also works in more serious settings, what do you think about it? Do you have ideas how to encourage some bad choices?