I hate just about anything and everything about MSOutlook, but this is one feature that I sorely miss in GMail.
In MSO, at least if you have the option turned on (although I think it's on by default), you start a new list simply by typing "* some text". When you hit enter, a new bullet-point is automatically created, and hitting enter twice without entering text ends the list. Likewise, typing "1. some text" begins an ordered list with the same behavior of the unordered list.
Lists are so common, especially for development and business in general, that this has saved me tons of time over the years. While I do get annoyed at most of the other automagic formatting that MSO crams in, this particular feature is gold.
To your question: yes, a MD-enabled editor would probably be the best. MD is quickly becoming ubiquitous, at least in my realm, and is a great compromise between an overblown WYSIWYG abomination and plain text.
That's because Outlook uses MS Word internally for email composition and rendering.
Earlier (Outlook 2003), it used to be obvious and also configurable. Now you're stuck using it with all its quirks and annoyances. (Yes, I'm not a fan..)
If I need a list of things in an email I'll do this:
Which gets me thinking about how cool it would be if gmail accepted and displayed markdown formatting..... Would that be a winner for you too?