There are only 11 units, and everybody pretty much knows everybody else. So when my downstairs neighbors blast their stereo at 3am, I have a vested interest in not completely blowing my top. I'm as nice as I can be because I live above these people and will continue to do so for a while. There would be repercussions if I told them where, exactly, I'd like them to stick their stereo. But online, there are rarely such real world repercussions.
People flame out over much smaller offences, and then just turn off the computer and go to bed.
There are often no real world consequences from virtual offences. Of course, this disembodied nature of the net also contributes to the stunning honesty and intimacy we find online. But online, safe, behind the glass of your monitor, you can reach out without fear and connect with another person. So the worst problem with Web communities is also the best asset. Riddle me that.