I wonder if the idea that you may be interviewing or be interviewed by any of the people you are conversing with means that one “behaves” better?
I am convinced that the combination of voting up good comments with strict, clear and fair moderation is an important reason. Add to this that reputation among peers may matter to people and maybe that is the formula.
I'm not so sure about the interviewing part. First, I am one of the few who does not use a pseudonym on this site, most do.
I have also seen people say some pretty absurd things using public Facebook comments that have their real name and details attached to them, I don't think people are considering that they may be making some career limiting moves when they get mad in comment sections. I also think that tech people have a lot more job options and freedom of movement between companies than other careers.
Sites like reddit that gather karma also create a "reputation" scenario for users, yet it still contains trolls, so I don't think it's that either, but it may be a contributing factor. That combined with the broken windows theory and the very specific target audience may be what make the comments mostly civil but certainly not immune to parody.
I am convinced that the combination of voting up good comments with strict, clear and fair moderation is an important reason. Add to this that reputation among peers may matter to people and maybe that is the formula.