I agree with BigNerd that dictating email style hampers free speech, and is ultimately an exercise in futility. So instead, my "magic wand" is pointed at myself: here's what I wish everyone did:

Acknowledge all emails. (Only where required - see below). Do it regularly (not instantly), even if just to say "I read your email and can't reply yet" or "I'd prefer to talk on the phone about this". That way, email goes back to being an assured delivery method, which is why it was so cool in the first place: you KNOW your recipient got your message.

One of the reasons myspace is winning as the preference of the next generation is assured delivery: you know that if you send someone a message, they'll see it. They might ignore or delete it, but you can (more or less) prove that they at least saw it, because you can see when they last logged in. And people act accordingly. Contrast that with email, where spam filters trap LOTS of legit mail, and plain old "email overload" causes people to throw up their hands and ignore / delete unread messages. When I send an email, I often have no idea if it was ever read, but when I send a myspace message, I'm pretty damn sure it was. Obeying a universal "law of replies" might bring that feeling back to email.

(And for the record, return receipts are no good for this - they're invasive of my privacy ... why should you know EXACTLY when I read your email? The acknowledgment has to be volitional.)

Three exceptions to the "Acknowledge All Email" rule:

1 - If it's a thread, I agree with Ed Eubanks that the last "Thanks." email is just annoying. My rule applies primarily to the first reply, where acknowledgement is novel. That said, it's polite and nothing to get worked up about.

2 - This rule doesn't always apply within an organization, where email delivery is already pretty much guaranteed, or with anyone you work with regularly enough to have confidence in delivery. If you're at that level, good for you. Still, a prompt reply (where warranted) is courteous.

3 - If an email requires no reply, I agree with Matt - the sender should indicate that by starting with "FYI" or some similar convention. We've got enough email in the universe, so if you can end a thread before it starts, do so.

And to chime in on two other topics:

Subject lines? Who cares - just parse out your tasks, get 'em into your system, and archive. All you folks complaining about bad subject lines ... it's because you use your inbox to implicitly store your tasks, isn't it? Come on, you can admit it, Merlin won't zap you (AFAIK). You're just piling that work on yourselves; parse the tasks out of an email once, and you'll never held hostage by someone else's bad subject line again.

Formatting? I'm going to go against the grain on the HTML email thing. If you don't want to read HTML emails, that's fine, but don't put the burden on me; run a script that pulls out just the text on your own machine. There's no inherent reason that text communications shouldn't include formatting like bold and italics; anything that helps convey tone is OK in my book. HTML is (unfortunately) the most standard option we've got, so until there's a better rich text standard that's universal, it'll have to do. (But I'll grant you that the prescriptive rule should be that you don't put meaningless formatting in the message - no funky backgrounds, animated gifs or comic sans. Use it to enrich, not distract.)