Good communication skills and the will to use them! The self confidence to be able to have hard conversations with underperforming players or guys with bad attitudes.
One of the adult rec teams I captain is terrible. We hold the leagues' win-loss record (and not for number of wins). But the guys have a great time playing together, and we've got a waiting list to get onto the team. The reason is that we laugh a lot - lots of jokes, the guys like each other, some funny joking scoring contests. A captain who can bring people together will do more for your team than anything else.