Another factor is that people who are highly active online develop a factual writing style that can be misunderstood.

Most people get unhappy if you respond, "Your mistake is ___." However online I might respond that way, and furthermore I appreciate it if other people respond the same way to me. This can and does lead to hurt feelings from time to time.

But that said, http://xkcd.com/386/ looks highly relevant and speaks to the dynamics you see online...