I think the main difference between HN and other forums is the moderation. I have been a moderator and I know what difficult and thankless task it can be. Thank you Dang.
Other forums are good and have moderators. But moderators absolutely make a difference, disproportionately to the amount of moderation actually done. Aside from overt actions, a moderator imposes a flexible but noticeable ethos on a group's debate.