As a youth coach, the thing that I'm really looking for is a good player who looks to get everybody involved in the play, rather than looking to pad his stats. Those seem to be the guys that everybody on the team respects, and that's the foundation for a good captain.
From my experience, in the younger age groups the captain has just been the one who scores the most points. It's pretty tough to ask a 9 year old to lead an entire team. So just give it to the best player and let him lead by example.
From say bantam and up, captain's should be picked upon mostly leadership skills. Should be talkative in the room and know how to get the guys ready to go for a game. Also must lead by example and it really helps if he and the coach communicate well.
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.
A guy that can lead by exemple and not be afraid to talk in the room and give some advice to other players. He also has to be a guy that will not take stupid penalties. Also he has to be able to talk to the ref without getting mad like some guys I see sometimes. The captain needs to be the guy that's willing to do anything to win. He can be a quiet leader ala Sakic and still be one of the best captains ever.
For most of my teams, I've turned down the title of captain. I don't need a title to be a leader when I can (and will) do it anyhow.
But for my teams that I coach, the title of captain is usually given to the person who is a leader that knows the game best (since they're the only people that are supposed to talk to umps and refs) and the person that is a leader and can deal well with conflicts between players.
I've been captian or assistant alot and the only thing i can say is dont try too hard to lead the team, just do the things that got you the letter and stick with that. I find that if I do my part on the ice and say the odd thing before a game teamates admire that much more then someone who goes out of his way to lead the team.