[blakeyrat]

If an end-user is using a markup language at all, you've already screwed up as a developer of usable software.

If you're going to use a markup language, at least use one that supports basic shit like underlining text, or typing the sentence "21." without it thinking you're trying to start a list.

No matter how users are interacting with the text on your site, do usability testing. If you do usability testing in good faith, and Markdown comes out as the winner, then use it, with my blessing.

Don't base your usability decisions on gut instinct or knee-jerk reactions. Which is pretty much exactly what every site using Markdown currently is doing. AFAIK it's never been studied in front of a fair selection of users.