I think there's a converse to the Greater Internet Fuckwad Theory—let's call it the Greater Reality Stick-up-the-butt Theory. Basically, in real life, we enforce social norms on one another that we don't actually see the merit in ourselves, because others expect us to enforce those norms and punish/shun us if we don't enforce those norms (think of the monkey/water story: http://freekvermeulen.blogspot.com/2008/08/monkey-story-expe...). Anonymity allows for the relaxation of those norms, because nobody's around to catch you not calling someone out for doing something that you would normally call them on, but which you actually see no rational reason to be calling them on, besides some vague sense of "propriety."

Basically, anonymity allows people to treat everyone around them as utter foreigners—you don't expect me to have any grounding in your culture, and I don't expect you to have any grounding in mine, so neither of us "needs" to be upset when the other does something that is uncouth. Because of this, discussion can concentrate more on facts, and less on signalling, which I (and I think many introverted people) find a less stressful mode of communication.

Tangentially, I think this explains why many people on the internet have a annoyance-trigger at the undefined name-dropping of specific locales within, say, the US (with the usual response of "we're not all from the US", even if the person stating that is, in fact, from the US)—it grounds the discussion in a specific culture, which increases the likelihood of some subset of the people in the conversation finding a valid means of conveying subtext—which then leaves others out of that subtext, creating a kind of I-can't-enforce-norms-I'm-not-aware-of anxiety which is projected into a problem with the context being introduced in the first place.

In a lesser way, this is also why people tend to overreact to women who identify themselves as such on the Internet: they're introducing an ambient cultural context that increases norm-enforcement anxiety. (Interestingly, there's a specific name for Internet-age gender-role-relative-norm-enforcement: "white knighting." It's quite common to see people calling the practice out, though usually even the people themselves aren't sure precisely why they're unhappy about it.)

There is less risk in being honest when I'm online. If I voice an opinion around my coworkers or family that they strongly disagree with[1], there is a chance that could change my relationship with them for the worse. Some people hold grudges for a very long time over minor things.

On irc, if someone decided to hold a grudge I can simply add them to the short list of people I've put on permanent /ignore. In real life I have to deal with them on a daily basis.

There is also the added benefit of irc having scroll back and delays in communication. I can read what I've said before so I don't forget what I'm talking about. I can also walk away for 5 minutes to grab a drink and cool off if the debate gets heated. It would be very difficult to do either gracefully in a live conversation.

1: Example topic: The death penalty.

One of my coworkers has voiced a strong opinion on putting to death anyone that kills another human being except in a clear cut case of self-defense. I disagree with his opinion in the strongest way but I keep my mouth shut because inviting a civilized debate over that topic is a good way to ruin an otherwise enjoyable lunchbreak. On IRC, I would be happy to invite debate because the cost of the debate going bad is far less.