David Pogue's blog: Whatever Happens to Online Etiquette?

Other possible reasons:

• The visibility of crazy (non-ordinary) people lowers the level of a room, such that ordinary people are already grouchy even when they are participating in conversation with other ordinary people. I've seen tight-knit online groups fall apart when a crazy person walks in and doesn't stop wailing. The ordinary people don't have to leave for civility to evaporate.
• Many people don't know how to be civil in text. We learn civility in real life from interacting with real people and picking up on many subtle cues. Those cues are mostly absent from text-based communities, and it takes much longer to acquire enough social data about an individual to know how to communicate emotional overtones effectively.

The unlearned lesson is to be extremely, unnaturally generous in online discourse, even in the presence of crazy, grouchy or uncharitable people.