In Outlook 2003, you could open an email message and convert it from HTML to text by hitting Ctrl-Shift-O. That was handy, because I generally prefer to bottom-quote, and that's a pain in the ass when the email is HTML. So when I installed Outlook 2007 I was annoyed that Ctrl-Shift-O didn't work any more.
After a bunch of fumbling around, I finally discovered that if you open a message and edit it (Message->Other Actions->Edit), you can then change it to plain text (Options->Plain text). Well, being a keyboard guy, I got pretty tired of doing that, so now I have the following key sequence committed to muscle memory:
Enter (open message)
Alt H X E (edit message)
Alt P L (convert to plain text)
Alt H R P (reply)
Which, honestly, sucks, but is maybe 100 times as fast as doing the same thing with the mouse.