The Christian Science Monitor explores why e-mails are often the source of miscommunication.
Though e-mail is a powerful and convenient medium, researchers have identified three major problems. First and foremost, e-mail lacks cues like facial expression and tone of voice. That makes it difficult for recipients to decode meaning well. Second, the prospect of instantaneous communication creates an urgency that pressures e-mailers to think and write quickly, which can lead to carelessness. Finally, the inability to develop personal rapport over e-mail makes relationships fragile in the face of conflict.
On the whole, the article is a useful summary of what you probably know already.
UPDATE: Kaimipono Wagner thinks these insights might apply to blogging as well.