I share the general enthusiasm for text/html. My own personal list of
things it lacks but needs for email use has two items. The first is the
"excerpt" facility, which Rhys already mentioned.
The second is the ability to establish hot links to other parts of an
enclosing MIME entity. This could be accomplished by an extention to
the URL syntax that allowed reference to the other parts. If you did
this, in conjunction with a new multipart subtype with semantics of
"only display the first part automatically, but make the other parts
available to the interpreter", you could make some very interesting
hypertext-in-mail things start to happen. -- Nathaniel