Opposing goals, the purpose of markdown is to have a nice looking plain text document that can be automatically typeset and asciidoc is to have a well structured plain text document that can be automaticly typeset.

Note that markdown's whole value proposition is that it is a nice looking plaintext format, it stinks at adding any sort of semantic value to a document. realistically if you care about your documentation you will use something better than markdown. The problem is adding better structure always makes the plain text ugly and hard to read, violating the whole point of markdown. that is to say, the many efforts to add better semantics to markdown don't appear to understand the point of markdown.