Great solution. Given the problem statements that lead to this, i was pleasantly surprised by the elegance of the syntax.

There are the obvious arguments about plain-text readability and additional standards, I would suggest thinking about this not as a Markdown replacement but rather a different use-case entirely. I understand the author framed it as a replacement. While that may have been the impetus to the project, i think the result is something entirely it's own.

Markdown has a place when a document stands to be published/shared in multiple formats. I use it regularly for blog posts that i also email to others for feedback in plaintext. Sundown looks to me like a light-weight, text-focused haml. Usable when something is intended to only be published in HTML, but where you want to be freed from that markup.