And the public upvote / downvote system make it even worse. Soon, you're reduced to a small subset of opinion because any slightly dissenting opinion will be downvoted to hell and hidden at the very bottom of the page.

Not necessarily an issue for small communities focused on a non-controversial topic (mostly anything about jokes/memes, or very niche technical stuff), but bigger communities focusing on slightly political topics become hell. The "europe" subreddit is incredibly toxic for this reason. Not because of individual redditors, but because of the group effect.

HN doesn't suffer from this problem both because of the niche aspect and because downvotes are restricted.