Edit: Just speaking for me, HN is next. Doubt I'll stick 'round much longer.
- no followers / following (no incentive to increase engagement)
- no notifications of responses to comments (maybe there's a way to do this with RSS but at least it's not obvious)
- no ads
- no endless feed
HN is much more like an RSS feed of interesting articles, where people can leave comments -- there is some back and forth (as in this case) but not a lot; it's centered around the linked content and thoughts on that content, not on engaging with other users. It's not monetized and therefore doesn't employ all the tricks that SM uses to "drive engagement", which is often driven by outrage (which is therefore a highly desirable component of a SM).