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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.
reinventing the wheel won't work unless you can show people that something is missing or wrong with what they're already used to. even then, it won't happen overnight, and it may not happen at all. take G+ for example. they made some not-insignificant changes to how people communicate in their network. they've already got a ton of users, but it's only been ~2 months. it could just be flashy-new, and could fade in another 12.
maybe another way to think about things is about what social networks represent: interactions between people. you have people you know, people you don't, people you want to talk to, people you want to listen to, things you want to share publicly, things that are private, etc. etc. maybe instead of building up a feature set, it would be beneficial to think about human interactions and relationships, and see how software may be able to represent that better than current models (G+ is trying this with circles).