However, the big plus of HN is the 100% absence of trolls (or maybe I just don't notice them here because they aren't encouraged). The most horrible thing on reddit is several people with technical competence who use it to abuse people - a form of trolling. I find that combination hard work to deal with.

I wonder if a factor is not "diluting responsibility" (psych 101) too much? HN is a smaller site (and has pg overseeing it); subreddits have fewer subscribers; /. has fewer (randomly chosen) moderators. Is too much dilution of responsibility what undermines social sites like Digg?