I was quite keen on SO at first, but have gotten much less so over the past year or two. The most recent incident that really put me off was a month or two ago when I found what I considered a pretty ideal SO question; ie. a "how to do X programmatically". Someone (a relatively new user) had already commented that it was a dupe of another question which it clearly wasn't; several of us simultaneously corrected him, and all seemed fine. Until a few hours later when some drive-by mod arbitrarily closed the question as a dupe; they obviously had enough ninja power to do that off their own bat, but it needed four or five of us to vote to reopen and the last I saw it'd only gotten to two or three. The original questioner seemed fairly demoralised by it; I wouldn't be surprised if he didn't come back.

I'm sure there are plenty of other cases of good moderation, but to me it highlights a serious problem with the site. One loose cannon (who I can only assume either was a badly written bot, didn't speak English, or was just trolling) and one overactive mod managed to ruin that question for everyone, and the rest of us had little power to really help.

I don't know what the answer to this is; it bears a lot of resemblance to Wikipedia, and they still struggle with it.