As with the telephone, e-mail is nearly ubiquitous, and most people use it very casually, without much thought as to its proper use.
In general, think of e-mail as analogous to a memo or a letter sent by FAX, and do not send anything that you would not send on paper. Also, take as much care in preparing your message for e-mail as you would with a paper message.
Be aware that there are still those individuals who consider e-mail to be informal, and prefer paper or fax correspondence. Etiquette means accommodating your audience. Even among those who use e-mail as their preferred mode, a paper letter often carries more weight.
Avoid digital gimmickry: weird fonts, colors, graphics, and the like seem cheap and cheesy. While the quality of physical stationery can create a positive impression, the same is not true of e-mail.