HN quality has certainly declined and honestly some areas of Reddit (even /r/programming) are quite comparable now.
But strong moderation and a culture that abhors the typical meme crap that fills other forums is certainly helpful in keeping HN reasonable.
Less cross pollination. I think of HN as a very specific subreddit, without the worry of having users from other subreddits 'brigading' as the cost to do so requires registration.
At reddit you can come across a post and easily make a pun, incorrect statement, or troll much easier.
In essence, I think what makes HN work is having had a very good "seed" of users and high quality contributions at the very start. Every new user is then peer pressured to only submit/post content of equal quality and will be punished if they don't (by being ignored/downvoted). But also good moderation. I am yet to come across a web community that works and doesn't have good moderation.
Somewhat by peer pressure. When that fails, moderation.
It seems to me like the model should be able to scale beyond just HN to improve the quality of discourse elsewhere on the web...right?
That depends on your idea of quality. Is the repetitiveness of humorless demands for peer-reviewed studies to back every word you write, and culturally enforced cynicism and contrariness, quality? 8 years ago, HN was 99% bright entrepreneurs having genuinely intelligent and polite conversations. Now it's an echo chamber of self-styled 'scientists' that reminds me of Less Wrong meets RationalWiki.
You joined less than a year ago.
Dwight Schrute voice False! I joined under my real name in 2009.
Posts/comments that are pure images aren't tolerated.