For me, a captain on a football pitch is one who is able to inspire the other players through his own example. A captain like Coloccini, for example, isn't one who is really vocal on the pitch but is one who leads by example (in Colo's case, it's his calmness on the ball, won't give up etc.). Such a captain should be able to inspire the others.
Kevin Nolan was a vocal leader and wasn't afraid to give his view when needed. He was able to keep the players together by his own example, sure he wasn't a proper athlete but he was dedicated to the cause and was a professional.
Now let's look at Michael Owen. He was quite frankly the worst possible choice for a captain because his example was not really doing anything on the pitch or even off it. For a player who really wasn't arsed with the cause of helping to keep us up nor was he a model professional is hardly the inspiration his peers needed. Great leadership material eh?
I agree, however in the Owen scenario it was clear that team didn't have anyone capable of been a leader.