I assume that many of the same reasons that make me put more effort into my HN comments than my reddit comments also apply to other people here. The net result

I assume that many of the same reasons that make me put more effort into my HN comments than my reddit comments also apply to other people here. The net result is that HN has a higher fraction of useful comments, and a higher fraction of smart, open minded readers. These reinforce each other, and keep the site viable. I think this is a key observation: HN started with a very slightly positive feedback loop for quality comments, and over time this built up to a considerable advantage. If HN ever collapses, it will likely be because some other feedback loop with greater gain drowns this out. My guess is that for most discussion sites (Slashdot, Digg, Reddit, etc) the desire for profit eventually prioritizes higher traffic over quality of discussion. My hope is that HN will remain mostly immune to this due to its (debatably) not-for-profit nature. [By the way, I thought your comment was brilliant, likely due to the care you put into writing it. To recoup some of the effort, you might consider putting some of the details of your background into the About section of your profile page.]