I am impressed that you came with such a list.
It is also obvious to me that plain text is the way to go. HTML is
tested and found to be useless after several years. It is very similar
to Java applets. They are not useful 99% of the time and reduces the
internet experience.
Somebody was talking about international characters. Well my native
language is not English, so I use non-English characters all the time.
There is no incompatibility between plain text and international
character sets. Your recipient is supposed to know in which language you
are sending the email anyway. if they are not, they probably doesn't
have the corresponding fonts mostly and it really doesn't matter. HTML
has more problems.
I always use plain text and I hate people sending html email. The only
ones I receive html mail are spammers. The rest of the html mail I
receive is actually plain text but sent as HTML. This is mostly done by
hotmail since IE+hotmail always default to html sending with no plain
text part. Try to read those emails with pine???