I will be honest. I am a forum junky. You can tell by how much I post. These forums may seem terrible, but they are some of the best I've ever used (nfl.com probably is the only one that tops this).
So definitely top 10, although it is still a hostile environment, especially in the Non-Game related forums.
However, I agree with your questions, they would be interesting to investigate. I find myself being waaaay more hostile on forums than I'd ever be in real life. Maybe the lack of face-to-face communication is the reason for this. It is harder to realize that you are talking to another human being.