Janet has a post up on communications between students and faculty. My opinion — as both a student who must communicate with faculty members and a teaching assistant with whom students must communicate — is that it’s most important to be clear, concise, respectful, and polite. You should always observe proper punctuation, grammar, and spelling or you’re liable to misunderstood. It doesn’t do much good if your instructor can’t answer your question because they can’t understand what the hell you’re saying. I have a few other comments below the fold.
One big concern for students is how to address the person with whom you’re attempting to communicate. Do you call them Dr. SoAndSo, Professor SoAndSo, Professor, or Bill? My policy depends on whom I’m emailing. If it’s someone I don’t know well or am emailing for the first time, I’ll go with Dr. SoAndSo. After that person replies, I’ll consider switching to a first name if they sign their email with their first name. This transition may take a while depending on the person’s prestige (yeah, academia is definitely stratified). Folks that I know (like my advisor, committee, and other faculty that I interact with often) get the first name treatment.

My biggest pet peeves? People who start their emails with either a “Hi” or “Hello” or sign them with “Cheers”. Call me by my name; unlike some people, I don’t mind if you wear it out. And if you’re writing to a bunch of people, give us some kind of name like “Dudes”, “Homies”, or “My Brothers From Another Mother”. I’ll let the cheers thing go if the person is British or Canadian (I expect that kind of stuff from non-Americans). Call it jingoism or xenophobia, but I think Americans sound stupid when they say “cheers” unless they’ve got a pint raised high. And it makes me think of the pair of poorly drawn cartoon characters on the left — I hear it as “Cheers, fuckface!”