I feel like I'm late in this discussion. Reading through the thread, I'm glad to see people have formed a more or less consistent general idea of what qualities a mod should have. Maybe I'll be repeating one or two things that have already been said, but I feel the need to voice my opinion, and I'll start with what qualities a mod shouldn't have.
I know from personal experience, and the experience of various friends of mine either from this or another forum, what a bad mod is like. It's that person who is a terror to all new people in the community and an unpleasant quiet menace to the old ones, that person who has attached themselves to an environment in which they have been granted superficial powers which they mistake for a truly important honor which automatically deem them better than any other people in that environment. It is that person who'll use their power of deleting posts, moving, locking threads and handing out infractions right and left with the slightest provocation, or even simply their bold blue name, to strengthen and preserve their status while discrediting people who speak against them. And they might even strike back! Why not, after all? They know they'll get away with it, so there's no problem.
I think it goes back to the way people are chosen for modship. I can't tell for sure as I've never obviously been part of the process, but I think more thought ought to be put into the question who gets the big bold blue name. Just because forum moderators are lesser staff, it doesn't mean the standards should be lower. On the contrary, they're the first in the line of sight of the average member who's active in a forum, the first to receive complaints, observations, suggestions, ideas. Their face is in their forum quite a lot. They are seen, they are recognized by most; in a sense, the posts of the moderator are important to people, valuable in their domain.
To land on my point, it's unwise to choose to make a mod of a person solely on their skill to manage one or many forums. That's not enough, in my opinion. The moderator needs to not only have good management skills, but to be a genuinely nice person with a sense of fair judgement. Why? Because it's readily apparent to members of that forum if they are not. It shows on their posts and on their comments if they are bad, and the good people are repelled, so that is a portion of people you are in danger of losing from that forum and possibly from the community as a whole, because of a wrong choice when electing the mod. Maybe I'm wrong about this, this isn't science or facts, it's what I've noticed. Good people tend to flock together, and if the head of the forum, who is unarguably an instrumental part of it, isn't one of them, they tend to find other places to hang.
Let me get to the qualities I believe a mod should have.
They are responsible for maintaining in their place a healthy, friendly environment and robust discussion. That much we all agree on, I'm sure. It takes skills in management and organization, people skills and leadership skills. Furthermore, to be a truly excellent mod, it takes a certain methodical, organized way of thinking and clarity of the mind. As some of you have already said, a mod is a person like the rest of us, and while my friend tells me that is such bullshit, especially here in PC, I have some good faith left; at least the staff is now starting to realize it and some of them echo that opinion.
You don't need to be humble, as a mod. Let's not place personality barriers, just because a person happened to become a mod, and let's not wave our humility card threateningly against mods, hoping to scare them away from wrongdoing. They will have to delete some posts, and they will have to lock some threads, and they will have to stand their ground and say no to people, because that's what they're chosen for. But if you are a decent human being that has donned the bold blue suit, you don't need to be humble in order to avoid being arrogant. You won't forget that the suit represents your duties to the forum you're modding, and you'll not abuse it intentionally, and even if you do it unintentionally, decency will require you to be brave and admit what you did was wrong. Dignity and correctness is the first trait we should seek in mods. Personally, another way I have of judging a mod is how approachable they are. If they emit an aura which pushes me away at first, if they do not respond to my messages or generally aren't responsive and actively involved in their own forum, I assume they are only fit for indirect management of their forum and will avoid messaging them with any problems I have, instead resorting to take my message to their superior - which would bring shame to me if I were in their position.