It's indeed not a nice situation, I'd love for my mailserver to insert a line into the message subject something like: 'TLS not used' when this is the case (or a plugin for Roundcube (Next?) that colors the subject, how cool would that be?). Just so I know and are aware of suspicious things. Enforcing TLS (and valid certs + strong encryption) is just not very practical yet, although it will definitely not affect mayor players' connectivity (Google, Apple, MS). Still, a lot of problems will arise, to see how many exactly, this is a rough indicator: http://www.google.com/transparencyreport/saferemail/

I predict in my group of friends I can receive/sent from/to almost everyone if I would enfore TLS on my server. Except to/from that one guy that is savvy enough to have his own domain but hosts his email at a cheap, crappy provider.