persistent identity and status, that is to say the 'social' part. A lot of damage of social networks arguably is rooted in the competition for status and attention that you get when you get twitter likes and people complementing you, and the horror when you end up in a shitstorm and your reputation is suddenly ruined. It's also what makes it addictive.
Most people on HN just seem to post more or less anonymously. The upvote or downvote mechanisms aren't even particularly visible.
The worst thing you get here is 4 downvotes and a heated discussion. This is not like twitter where your career and face is on the line.