Nagin was elected as a reformer, a no nonsense business oriented Democrat who spoke the plain truth. I probably would have voted for him the first time, at least if my other option was Marc Morial, the former mayor. But there's a difference between being plain spoken and showing your ass on national tv, when the last thing that was needed was heated tempers. And his ensuing re-election campaign consisted of some pretty undignified race baiting, since he knew he'd need the black vote to have any chance. I think he would have gotten the black vote anyway, being the only black candidate (remember, the first time he won it was against the also-black Morial).
With two bad candidates to choose from, maybe New Orleans took the least bad path. But I don't think it's going to make recovery any easier or more complete, either physically or socially.