This is by no means an all-inclusive list.
First - Avoid sloppiness: it's unprofessional. Poor grammar, spelling, and sentence structure is no more acceptable in a business email than it is in a letter or memo. Check twice for errors. Your reader will appreciate the effort.
Second - Capitalize the first letter in the beginning word of a sentence and in all proper nouns. The rules haven't changed on this. Using all lowercase letters is a shortcut attempt to indicate a conversational informality. The fact is, it doesn't work, it doesn't look right, and it can leave a bad impression. A well-written email can carry a conversational tone without breaking the rules.
Third - Avoid shouting; you can be heard without raising your voice. Using all uppercase letters in a business email can appear pretty abrasive, like somebody screaming, even though it is often intended simply to emphasize a point. Proper emphasis can be achieved simply by writing well.
Fourth - Use abbreviations and acronyms sparingly. The Internet has spawned a large vocabulary of these, but be careful. There is always the risk of confusing your reader. Acronyms, in particular, fall into the category of jargon, which is fine if they are terms your reader understands. Abbreviations intended to speed up the reading process often slow it down by impeding comprehension, which is a quick way to alienate your reader.
Fifth - Avoid using emoticons; they're too cute for serious business email. Besides, there are too many of them and your reader might not understand the meaning of more than one or two. By way of illustration, click on this list of emoticons. If an emotion needs to be expressed in an email, it can be spelled out. If you are disappointed or happy with last quarter's earnings, it's easy enough to say, "I'm disappointed" or "I'm happy."
Sixth - Answer your email. Even if all you have time for is a quick, one line response, your reader will appreciate the effort and, in fact, is expecting it; it's a professional courtesy that tells the sender that he or she is not being ignored.