I think community management and content are both responsible for creating the kind of community Flickr. Vimeo, Veoh, Viddler, and most other video sites all have more mature user communities. This might be a function of size, but the fact that rival video survaves usually host user-created videos instead, and therefor have more invested in the site might have something to do with it.

Also, we shouldn’t underestimate the role demography plays. Sites like Flickr and Facebook cater to more upscale users, whereas sites like Youtube and Myspace attract users that are more low-rent. In a way, Youtube is the biggest trailer park in the universe.

Many of the other comments I agree with but there’s one thing that hasn’t been mentioned. I think the character of any social community is going to ultimately depend on the nature of the core reason people are getting together in the first place. Photography is a more contemplative art, a freeze frame of life. It encourages people to stop, look, and think. Video is seen as more disposable probably because after you’ve watched it, it’s done. It doesn’t stay there like a photograph. I think this makes it easier to dismiss a video or even the person making it in the comments.

I wonder what you and others would think of a social website that has a very, very long standing and established real life community like ravelry.com.

So I would exactly say that it’s the management tools. The issue is that the system encourages it. Spam and dumbing down the discussion is the easiest way for people to make their video popular.