I've wondered about the viability of text/markdown for email from time to time. It would make a very nice sensible default: the sender can write an email with a hierarchy of headers, quoting (exactly the same way done now with text/plain and supported by most clients), emphasis, hyperlinks, lists, etc., all without depriving the receiver of their preferred formatting and font settings. Also, not nearly as verbose as HTML (and not nearly as much crap).
