I think the difference is more that the people on HN have a lot more shared culture and shared values, meaning many are willing to make the same assumptions and

I think the difference is more that the people on HN have a lot more shared culture and shared values, meaning many are willing to make the same assumptions and see the same outcomes; while the people at reddit are more diverse and many are starting with totally different assumptions which would make your argument invalid. Also, I think people tend to link their HN profiles more with their real lives. They shed a little more anonymity to gain a little more credibility. On Reddit people are a lot more anonymous, which makes it easier to brutally attack someone because there aren't many repercussions. In general I am happy that there is almost always a dissenting opinion in the comments on HN, pointing out flaws in the argument that I might have been too complacent to catch.