A community is only as good as the quality of the moderation. HN is unique in that it has very dedicated people who read every comment and take time to explain why a comment isn't productive to the conversation. That minimizes trolling/dunking, which has sunk countless other communities.

I've also noticed that other message boards usually become "productized" once it has a large enough audience, and that tends to drive down the quality of conversation as well. Since the HN forum is an offshoot of YC, that hasn't happened. Well, I guess HN could technically be considered a job board for YC companies, but those postings are dwarfed by the regular posts.