Is it simply the "you won't be able to hit me across a computer screen" thing? Or does it have something to do with not associating a virtual persona with your own "self"?
In my experience people tend to listen more when the conversation is held in person, and seek to understand more. Whereas on the internet, my experience has been that people simply use the internet as a tool to "get their voice out there". Because there are so many views to be heard, we get used to ignoring some voices and responding with hostility to others, without seeking to understand what the other person was really saying first.
Sure, there are always trolls. But what worries me isn't the trolls who do it for the lulz. It's the people who are genuinely responding thoughtlessly without regard for whether or not it's appropriate.
I'm starting to think that the internet and fora in general aren't effective communication media, or that because of the nature of interaction (anonymous), the quality shared is necessarily limited because there is no benefit to the individual to contribute.