Many social sites start with a small community of thoughtful, intelligent people because they were created by those people. Certainly this is how reddit started, and HackerNews after it. Then as the site becomes more popular, the masses (and particularly the immature masses) join up. As the masses join up they start outnumbering the founders, and the center of gravity of the site moves towards the lowest common denominator. This makes the site even more interesting to the masses, who join in greater numbers, and so on.
Current sites are on different positions on this timeline right now. At the far end is HackerNews. Reddit is more devolved -- because it's older and because of the digg debacle. Then probably comes digg. At the far end of the sewer of the masses is 4chan. But make no mistake: all these sites are gravitating towards 4chan-ness. It is unavoidable. Our only hope is that as sites slip towards oblivion, new ones take their place at the top of the hierarchy.
I suspect the #1 reason why these sites devolve is because their handles are anonymous. This gives you leave to be a jackass where you'd never do that in reality.
I have decided to test this. On reddit or digg or whatnot I have my own name as a handle for official announcements, and of course I have various anonymous accounts, including novelty accounts. I'm sure that's the case for everyone here. But on HN I solely post under my own name, and have no anonymous accounts at all. Numerous times I'd write some snarky thing on HN only to delete it at the last minute as I realized that this was going out under my real name. As a result I think my comment quality has been radically better and more thoughtful than it has been on, say, reddit.
I still think the flow is unavoidable. But I wonder if HN could at least slow the inevitable flow towards oblivion by requiring real names.