> there's a far more positive community when people have a shared interest

Even old fashioned web forums and email listservs are still thriving in some niches and work this way.

I think reddit fails here because it's too frictionless. Requiring a little bit of intentional action to join or subscribe keeps out a surprising amount of drive-by trolls.

HN is pretty big now and has its share of trolls and bots but because it doesn't use social media accounts it's not nearly as bad as Facebook or Reddit.