This piece explains why flaming in online forums (email, usenet, blogs, etc) is so much more common than the in-person kind: it has to do with the inability to gauge reaction and adjust. Yes, I realize that it's obvious :) Having said that, it's a good read:

Communication via the Internet can mislead the brain's social systems. The key mechanisms are in the prefrontal cortex; these circuits instantaneously monitor ourselves and the other person during a live interaction, and automatically guide our responses so they are appropriate and smooth. A key mechanism for this involves circuits that ordinarily inhibit impulses for actions that would be rude or simply inappropriate -- or outright dangerous.

In order for this regulatory mechanism to operate well, we depend on real-time, ongoing feedback from the other person. The Internet has no means to allow such realtime feedback (other than rarely used two-way audio/video streams). That puts our inhibitory circuitry at a loss -- there is no signal to monitor from the other person. This results in disinhibition: impulse unleashed.

This relates to the "tone" post I made last week - and explains why it's a bigger problem online.