You missed the big one: engaging content for people to gather around. MySpace was music/photos. Facebook is photos/status updates/notifications. On Hacker News it's submissions and comments.

Hacker News is a social network and it's kind of the Craigslist of social sites. It doesn't get much less featureful than this. Which proves a vitally important point: Highly Engaging Content is far more important than any bell or whistle. Not to say that HN couldn't benefit from a few select additions though, messaging is an obvious example.