It's easy to fire off an email with barely any effort at all. And that's exactly how much effort goes into most emails: none. Ole Eichhorn's Tyranny of Email offers a succinct set of guidelines to avoid thoughtless email abuse:
Before anyone posts a comment, hey, how about that Lifehacker’s Guide to Weblog Comments? 
http://www.lifehacker.com/software/top/special-lifehackers-guide-to-weblog-comments-126654.php
Its also a good idea to assume that everyone may read it, considering how easy it is for others to forward mail around.
And in the financial industry (where I work) the Sarbanes-Oxley act pretty much assures that your mail will never be deleted.
The 43 folders advice is interesting because except for point 5, it’s the same “inverted pyramid” style that newswriters use. (One presumes that the authors of news articles don’t ask for what they want. Perhaps they expect us just to infer it. 
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Writing advice is pretty much universal, and just coz it’s email doesn’t mean the same rules don’t apply …
Good business writing may have some broad similarities. However, I think there are some major differences in how you write an email, a blog entry, and an IM…
Writing advice is pretty much universal, and just coz it’s email doesn’t mean the same rules don’t apply …
Oh, really? So writing an IM or a blog entry is just like writing an email?
Not exactly alike, but very similar. Business writing style guidelines aim for clarity and effectiveness; the same basic concerns are always present.
Are you being contrary on purpose Jeff?
Here is Guy Kawasaki’s guide on this topic, “The Effective Emailer”
Something I’ve done in the past is set up a rule in Outlook to hold mail for [x] number of minutes before sending it…this has saved me in the past as on more than one occassion I’ve asked myself…“is that message really email worthy”, or even saved my ass from sending a heated message to someone.