HN is heavily moderated, and it has a nerdy subject matter that selects for a certain type of audience. Those both help, but I think the biggest thing that makes HN so great is its peculiar business model.
I don't know if YC keep this site running purely out of love or if it's a calculated ploy to make money, but if HN is profitable for YC then it's in a very indirect and hard-to-quantify way - e.g. YC startups using HN as a recruitment platform. Dang doesn't have money-hungry VCs breathing down his neck telling him to maximize "engagement" - the day that happens will be the day HN devolves into an unreadable, unusable pile of garbage, like what happened to Reddit.
There aren't many communities like HN because there aren't many organizations that have the resources to run a site like this without running it into the ground because they have dollar signs in their eyes. The few that do exist are probably small forums focused on a super-niche topic or hobby, but if I knew of them I wouldn't post them here because I wouldn't want to spoil the secret.
Moderation is such an interesting art. There is moderation like trimming a bonsai tree... the effort is to keep everything healthy and beautiful, but not to avert the nature of the plant, to embrace its natural growth... then there is moderation like on reddit, which is politically and advertising motivated to force the community into a given shape, like trimming hedgerows. Either step in line or get pruned.
If anything HN feels like it has a lighter touch with moderation than most other places online, and I have never seen them outright censor certain political views or show any sort of favoritism towards any one company or group. I think much of that comes from having a small, passionate community that can manage the task of behaving and debating in good faith.
If someone could figure out the method for crafting communities like HN for other interests... that person would be a treasure to the internet.
I don't know if YC keep this site running purely out of love or if it's a calculated ploy to make money, but if HN is profitable for YC then it's in a very indirect and hard-to-quantify way - e.g. YC startups using HN as a recruitment platform. Dang doesn't have money-hungry VCs breathing down his neck telling him to maximize "engagement" - the day that happens will be the day HN devolves into an unreadable, unusable pile of garbage, like what happened to Reddit.
There aren't many communities like HN because there aren't many organizations that have the resources to run a site like this without running it into the ground because they have dollar signs in their eyes. The few that do exist are probably small forums focused on a super-niche topic or hobby, but if I knew of them I wouldn't post them here because I wouldn't want to spoil the secret.