In many ways, email makes it too easy to communicate. It also makes it easy to pass along someone else's words inappropriately. A pet peeve is forwarding email without asking. Especially provoking is when email is edited, then forwarded. Sometimes phrases are taken out of context, but the original signature is left in place.
People are outraged when they feel their words have been twisted.
Other times it isn't how you say it, but what you say. At UC Irvine, for instance, a heated debate broke out a while back when one department emailed much of the campus about its charity auction. In electronic jargon, to flame is the email term for sending angry messages when provoked by the ill manners of others. However, flaming itself is considered boorish. I think it's a little on the technical side to know that all capital letters is considered shouting in email.