> I wonder if it's less about metrics and more about principles; how does HN keep the level of quality so high? Is it because we share an interest? Because of the quality control? The UI/UX of the site? Something else?

HN is successful because it can be moderated by one dang. Maybe a bonus helper can appear.

But once you get so big that your dang can't do it all anymore, it will eventually fall apart.