Few months ago, I posted an article about the misunderstandings and confusion that often occur when communicating by e-mail.
Because a picture is worth thousands words, here is a pie chart resulting from research done in the sixties:
The percentages describe the importance of the effect an attribute has on a the receiver of the communication (in a work environment). Because body language (non-verbal) and tone of voice are definitely not conveyed in an e-mail, it easy to see why e-mails are easily misunderstood (SMS is worst, but that's another story). As a byproduct it also justify the existence of smileys.