See my adjacent comment to yours, I feel like social networks that try to jumble users together indiscriminately of their personal interests are doomed to grow to this amorphous mass of disagreeableness that reddit turned into.

The naive approach to solving this would be to create small communities that can interact between themselves as part of a larger network. I'm not there with my project, but I'm getting pretty close. I hope that it will prove to be the correct solution. :)

HN gets more activity than most reddit-likes, but it stays pretty tame compared to most others.

But it also lacks the general appeal that reddit has.

That's cynical. Follows that you couldn't have large (reddit-sized at least), good-natured platforms. I too see the tendency, but I think it will be solved.