[gizmo686]

This thread would seem to be a counter-example. The top of the thread looks much better than the bottom. As far as I can tell, this was done entirely without moderator intervention [0]

[0] Dang does have a comment but, if I understand the meaning of [flagged] correctly, it was user downvotes that triggered the killing, and Dang just provided commentary. There is also a perfectly comment marked as [dead], which I assume is a shadow-ban for an unrelated matter.

[guildwriter]

Not exactly. HN is a community that is relatively small and has a strong culture. This is one of HN's greatest strengths in preventing the same sort of issues that occur in reddit. Larger subreddits show the breakdown of the upvoting system quite dramatically. Subreddits that become heavily polarized also suffer from this. You can see HN having the same problem in heavily contentious topics or flagged posts. This is where the left leaning majority demonstrates it's influence the most. Comment quality hits the skids right off the bat and never really recovers.

Inevitably it is the culture of the posting community and the adherence to that culture that allows for this kind of thoughtful posting to occur. Moderation is a fundamental requirement as well. I've seen many instances in large subreddits that experiment with relying on the upvote system that end in total failure. For a time the community policing works, and then the front page is dominated by low effort content and shit posting.