Or maybe people are just people, pleasant and unpleasant and everything else, and it can't really be helped?
It seems like putting something on the front page like "hey don't be a racist busy-body" isn't really enough. And at the other end of the extreme you've got Facebook employing organisations that filter sensitive content - with fairly horrific outcomes for the individuals who do the monitoring.
HackerNews is a pretty good example of getting a certain standard of behaviour out of people en masse. I wonder what the secret sauce was?