Guy Kawasaki has a post on email ettiquette. Here are my main bug bears about email and etiquette. Kent Newsome riffs on the Plain Text rule. 
People who send me big files. I read much of my email via Gmail on GPRS. If you can't fit it in text, don't send it. Alternatively, provide a decent description of what you are sending me, and make it as small as possible.
It's all too easy, in this age of broadband, to think that the world has lots of bandwidth. I do. I have a 2Mb connection at home. But when I'm on the train, I have a very, very slow connection. Deal with it.
Cut your quotes right down. Top-quote, but make sure the quote is relevant. If I'm reading on my phone, I don't want to have to read through my whole message to understand it. I also don't want to have to go right down to the bottom of the message (read: browse through three or four screens on Gmail Mobile) in order to understand the context.
HTML email is usually spam or marketing crapola. Most, if not all, of it goes either to the Spam bin or the Trash bin. Don't follow them there. Send me text. Send me small amounts of it. And give me context.
Oh, and cut all that corporate legal crap off the bottom. State facts and/or opinions. But don't state them then deny them in your sig. That's horseshit and you know it.
