I think that at some stage the default email format should be changed from "Plain Text" to "HTML". Personally, I use HTML email both on this install and on our private Phabricator install and have not seen any real issues (we also have inbound email setup on our install). In my opinion, HTML email provides a much better user experience.
I'm just going to merge this into T992. I want to complete stuff like T6600, T6576, incomplete markup of field sections ("Revision Summary", "Test Plan", "Inline Comments"), T6670, blockquotes having weird rendering, etc., before making this the default.
HTML mail is probably better overall even right now, but also clearly broken in a lot of ways. The current UI sets that expectation appropriately, but I'd expect users to reasonably be confused by the various obvious broken aspects of the mail if we defaulted it on.