Could that just be a size thing? I remember Twitter being a lot more sane place before it was overrun by politics a few years ago. Or maybe the algorithm was better at keeping that stuff away?
Reddit, slashdot, and a whole slew of others went through an HN-like phase of high-quality discussions and content before turning into cesspools.
I think what makes HN unique is benevolent sponsorship. There isn't pressure to grow. It's recruiting for Y-Combinator, which eliminates most of the normal pressures.
* Archived here: http://atdt.freeshell.org/k5/story_2009_3_12_33338_3000.html