I think you are setting the bar a little too high. It seems to me like there is quite a bit of mail that benefits from HTML. Superscripts, subscripts, italics, bold and colored text (used in moderation), inline images (used in moderation) can be really useful when discussing concepts that aren't easily reduced to characters.
Suppose your email would benefit from some mathematical formulas. You can do the old standby and drop into latex math mode, but the person on the other end of the email might not understand what you mean by
\sum_{n=1}^k\,\frac{1}{n} \;=\; \ln k + \gamma + \varepsilon_k < \ln k + 1
much easier and clearer to use LaTeXiT or something similar and copy/paste in an inline image with the formula correctly formatted.
Or, say you are a taxonomist. Italics in species names are not just a stylistic choice, they also convey additional meaning. For example in
Epilobium ciliatum Raf. subsp. watsonii (Barbey) Hoch & P.H. Raven f. rosa
the italics show what parts of the full scientific name refer to the species and what parts refer to authors or levels of taxonomic organization (subspecies and form). You can't do that with plain text (you could of course do it with markdown, though)...
plain text is sufficient of course; but sometimes the additional bells and whistles of HTML really are useful.
I generally think /italics/ and bold, quoting "> ", ">> " etc work fine. In addition to utf-8 encoding, you've covered a lot of ground (for English-speakers, utf-8 might seem like a luxury, and of course, if you demand unicode support you're beyond "basic" plain text -- it is however what I mean when I say I prefer plain text emails).
Formulas an illustrations can usually just be appended (and while they won't be shown in-line most clients will display images (and those choosing clients that don't won't really complain), but yeah, if you need multimedia you need multimedia.
Now, I don't really see how an image of an equation is really enough -- if you're working with someone, you'd want them to be able to quote you, reply to you -- and most importantly, tweak your work (edit your equations). I'd argue such (genuinely rich documents) don't really belong in email. Use a wiki or something (and then you can email wiki-markup...).
In short, I'm not convinced all the down sides and added complexity of html mail is worth the hassle.
Are rich documents and hypertext (hypermedia) a good idea? Yes. Does it imply a truly object oriented system, essentially mailing each other runnable smalltalk code? Yes. Will that be secure? No. Will that be standardized? Not by the looks of things. This is essentially why office suites are a source of security holes and incompatibilities. And web apps (though differently).
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!
