Yes I have been a forum moderator. Both here and elsewhere, using a variety of different pieces of software, on a volunteer and paid basis (as part of a sysadmin role for a few different companies).
The why is a tough question. It's not exactly "great" being a moderator. It is work. There's tons of procedural stuff--moving, merging, locking, renaming threads. Dealing with account issues (which are very common, lots of people get locked out of their accounts). I've got 3750 PMs right now, and that's after pruning a thousand or more last year. Normally I get a bunch of work every day.
I don't get a lot of pleasure out of banning people. It's an unavoidable consequence of a larger forum. You want the forum to be accessible to everyone. You don't want it to descend into bickering, cliques, wars of forum personalities, spam, racism, etc--and because some people simply can't learn to respect this stuff, and some people simply can't learn to let this stuff go (IE people "take the bait"), the quality of discussion is lowered. Sometimes warning or posting in the thread is enough to set an example, especially if you get in early, but sometimes there's no recourse beyond locking the thread, deleting the posts, or banning a user. I average about one ban every two days.
The best part of being a moderator anywhere is the idea that if you have a really strong vision of how discussion could be better, you can help realize that. In my case, I try to encourage people explaining themselves, avoiding empty "+1 qft i agree my #1 game is unkarted 2: drake's powerslide see spot run run spot run" level stuff. I try to steer threads back on topic. I try to ask probing questions or answer probing questions. I don't have a blog or anything so when I watch or play or read something, I tend to post about it here. I also try to PM users who are doing a good job to thank them for contributing good content.
Another thing I did as moderator was to, without really getting into any details, work on making it easier and more convenient for moderators to check user's past ban history so that we could get better at giving lighter sentences for first bans and heavier sentences for problem posters... and try to avoid letting users get away with 15 bans for the same thing without being permed. This also has the side-effect of trying to standardize ban lengths a little bit more. When I first became a moderator on GAF, the criticism that I read the most about moderation was that it seemed scattershot or random. I can tell you that's emphatically not the case anymore. Moderators all talk to each other, bans, especially bans that are lengthy or that might be controversial, get bounced around with various mods giving input. There aren't any loose cannons anymore. That's not to say everyone is 100% in agreement about everything, but just that we work together as a team. And by and large, the decisions now are better than they were before. I like to think that even if a poster doesn't agree with our rules, they'd largely agree that we apply them with a good amount of consistency.
The criticism I hear most commonly now is that the forum is too sanitized, or too bureaucratic, or not wild enough. That's a product of having almost 100k users. Posts that would have been hilarious and wacky at 5k users derail threads and wreck discussions now. If people are looking for a forum that's a little more off the walls, I totally understand that. There are plenty out there. Different forums are better matches for different people.
If I didn't think I was part of a team that helped make discussion better, I wouldn't be a moderator. None of the "perks" give me any pleasure. I don't want sycophants or fanboys or suck-ups
edit; also, does gaf have an irc channel? i wanna get in on that. I miss chatting it up on irc.
irc.gamecubecafe.com #ga -- not an official NeoGAF IRC channel (not the same rules, not the same ops, overlapping userbase but there are non-GAFfers and GAFfers who don't post there) but closest thing to what you're looking for.
