Seems like a good idea, however I think it takes Markdown only a step in the right direction, without going the full way — allowing the user to specify custom "tags" and allow for custom styling of those tags (with html/css or some user friendlier way).

Basically, LaTeX, but:

- web ready, no pandoc tex→html mangling

- user friendly, allowing for 0 customization and still giving nice results

- with nicer markdown-like syntax for the common things, and a non-obtrusive way to include custom tags

- with an easier/better language behind it (not just macro expander)

I'm still just a student, but I frequently find the need to use something other than just math/list/image/link, e.g. theorem&proof blocks, Q/A blocks, you name it. The thing I'm currently using is Pollen [1], but it doesn't come with common tags defined and so the initial setup is a bit of a hassle.

[1]: http://pollenpub.com