It would be very nice if the message-composition window
were able to support WYSIWYG HTML-based messages.
Something like "htmlArea 3.0" from
http://htmlarea.com/, or the one from
http://www.kevinroth.com/rte/demo.htm or any of a host
of other alternatives.
Basically, the "message" textarea would need to be
replaced by one of these "plugins", and a plain-text
(tag-stripped) version would need to be generated as
well (for the plain-text MIME-type).
Obviously, this should be optional since 1) many
"purists" believe that HTML-mail is evil, and 2) some
browsers (notably Opera, Konqueror, and Safari, from
all accounts) lack the functionality required to make
this work. So a global "Enable/Disable HTML
Composition" option should be provided, along with an
enable/disable button on the composition window itself.
Also, message should be sent with both the HTML and
plain-text versions, so that recipients can have their
email-clients configured to look preferentially at one
or the other.