My default for gmail is sending plain text, but sometimes I will flip to html for bullets or if I am sending a project plan or some similar that I want formatted nicely.
But yeah, most of the time text I think that plain text works better.
(I suppose I could use some crazy utf-8 characters, but I generally don't).
More to the point - if layout is important, I'm more likely to mail someone a pdf - but I usually don't because I usually expect people to read mail in their mail reader. And that means respecting however they prefer to read mail (eg: background/foreground colour, font, quote-styling etc).
Then again, that used to be the way of the web too -- and now all browsers have basically given up on user stylesheets (and we all know we need to "reset" the CSS for a baseline for our "fancy" web page layouts...).
All that said, I'm not horribly against simple html in mail: No css, em/strong, h1..6, in-line images (with images bundled with the mail for privacy and off-line reasons) -- basically "rich text". But with a text/plain part!
But yeah, most of the time text I think that plain text works better.