I had a thought re: site quality -- which is that a community ends up with a focus whether it likes it or not, as the community hardens and desires a regular feedback loop -- so while Digg and Reddit opened themselves to a "lowest common denominator" and reaped what they've sown....YC started with a focus on hacking and startups. Which is good for YC content, since it means the people who are interested in it are predominantly optimistic, thoughtful, or both.

Lambda the Ultimate is another good example of this; even as it's gained popularity, it's preserved an agenda that avoids language wars and other noisy content. I would say that because of its narrow focus, its quality is even higher than news.YC!

One online community which has imposed some standard of quality from above, without topical limitations, is the Something Awful forums. The strategy there is to use fee-gating and heavy moderation with frequent and somewhat inconsistent punishment. The results are dubious - for some posters it becomes a game to see what one can get away with, and the overall quality is only increased in that a larger percentage of posters take time for spelling and grammar, and obvious trolls get banned, but clever ones do not. Basically, moderation treats only some symptoms of an underlying problem: people treating the online world as if they were sitting down at the bar with 30,000 of their best friends.

That said, the worst elements of YC in the long term will probably correlate to the worst elements of the startup world.