It seems on forums that a whole different side of people comes out and any mention of left/right or God/atheism etc triggers flamewars.
There are two main factors, I think:
- anonymity (for many of us)
- lack of face-to-face communication
Something about anonymity makes people drop some civilities, and lack of face-to-face communication makes it very hard to pick up on cues from body language or spoken tone that can indicate the mood of the speaker. A comment that would seem perfectly innocuous in speech can come across as extremely hostile. In other words, when we speak to each other, it's easier to know how something is meant, whereas in written communication, most people tend to interpret others' words not in the others' mindset but in the reader's mindset meaning that a fair bit is lost in translation.