Well, I'm no 'otherwise' psychologist, and one a little amateur one. My best should would be... Well, to put it simply, most people are plain afraid of receiving negative emotional feedback (take out some masochists and such) - and web is a perfect tool for avoiding it. You don't see hear the voice, the intonations, nor do you see the facial expression and body language - all those minor, yet important ways to add emotional to the rational of a pure word.
Another thing - life imposes its 'technical' limitations, as such: you have to be fast. You don't have an evening to sit comfortably and think of a witty reply to some smartass from the forums - you have to think of something right now, and that's not something every person can do easily.
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Observation: Many forum topics at some point degenerate into name-calling, questioning the other's manhood, etc. This is an unfortunate, though interesting phenomenon that I believe is related to these similar observations:
From the safety and relative anonymity of their automobiles, many people will make rude gestures, or worse, at the merest provocation. (These same people would probably very kindly hold the door for you if met face-to-face.)
In office settings when using e-mail, people often will say things they would never say face-to-face. Talk to the same person on the phone or in person and they are usually much more agreeable.
I guess the common theme is that some people, when safe inside their cocoon, behave differently than when outside it. Any psychologists out there, amateur or otherwise, who would care to comment?