I've been waiting for someone to build this... I occasionally googled "markdown track changes" in the hopes that something turned up.

My need for such a service stemmed from working at a full-service agency, where website copy would be written in docx, sent to the client for review by email attachment with track changes enabled, then eventually on to the coders to flow in to the site.

It would be so refreshing to receive copy in markdown format instead of poorly utilized docx files! (It drives me nuts seeing headings done by bolding and increasing size...). In the meantime I wrote a decent XSLT to transform docx to markdown... But ideally everyone in the chain would be using markdown.

If Draft could make this type of workflow more appealing than passing around docx files by email, I'm sure you'd get many agencies jumping on this service. Good money to be made there.