Room to Goof Off . These groups almost always leave room for play. Sure, someone's got to finish that coding, but she really wants to take out a couple hours to play a game. A bunch of people might wander off to grab food. Sure, class needs to be structured to some extent, but a good joke is a good joke and it really can liven up the place.
People know when to goof off, and it helps in the long run.
Leadership Through Respect . There's hardly ever a need. The others usually defer to the leader's judgment because either it's given with sufficient reason or because there's sufficient trust that the leader has everyone's best interests in mind. And the leader respects the talents and contributions of everyone in his or her team. Even the quiet people who work in the background are acknowledged and respected.