Just an historical point on text/richtext. I think (perhaps incorrectly) that
I played a significant role in getting richtext dropped from the original MIME
spec. While working on BBN/Slate, a multimedia mail system of CMU's Andrew's
ilk, I tried to implement text/richtext input/output converters. I found that
1) it had many Andrew-based predjudices in its text model that did not jibe
with the model in either Slate or your typical Mac/Windows simple text
processor and 2) there was an amazing amount of implied semantics that came out
of Andrew's handling of formatting that was not in any way part of the spec.
In addition, it had "the kitchen sink" including
headers/footers/footnotes/floating figures/keep with next, etc. that was
entirely inappropriate for a common denominator rich text format.
text/enriched is much more tightly specified and much more constrained in the
kind of markup it is trying to provide.