I'm not entirely sure it's about content (while HN is certainly tech-focused, politics, health, philosophy all come up with regularity) or even content moderation, although they both certainly play a part (particularly the moderation around here. Thanks, staff!)

I wonder if it is more to do with the community itself. HN users tend to have very intelligent discussions on pretty much anything, and discourages shitty, unnuanced, one-line takes. This, coupled with a healthy moderation system, makes it hard for the lower quality discussion to break in and override the good stuff.

The car headlight forums seem to expose the weakness of small web though, in that a lot of the forums that show up in search are "sponsored" by one or two major brands and any open discussion or validation of off-brand solutions, AliExpress parts, etc are quickly shunned or banned.

Yes. That's the small web.

A good example of the generalization problem you discuss is reddit.

You have to unsubscribe from all the defaults and find the small, niche, communities about specific topics. If not, it's the same stuff, reposted, over and over, across different subs and/or social sites.