This is a placeholder for a more complete TechCom-RFC. At E212, @RobLa-WMF volunteered to take some of his existing writeup on this subject, and post it in a (very rough) RFC.
The result:
https://www.mediawiki.org/wiki/Requests_for_comment/Markdown
Markdown support isn't intended to replace MediaWiki markup for all uses (especially on Wikimedia sites), but the proposal is intended as a (hopefully) interesting thought exercise during the summer travel season and as a hallway-track discussion topic during Wikimania. Related discussions: T127329, T112999#2377981
Markdown is "a lightweight markup language with plain text formatting syntax designed so that it can be converted to HTML and many other formats using a tool by the same name" (https://en.wikipedia.org/wiki/Markdown).
IRC discussions