HN quality has certainly declined and honestly some areas of Reddit (even /r/programming) are quite comparable now.<p>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.<p>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.
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.