Actually, Docbook started as an SGML vocabulary and only in version 5 became XML-first. And SGML, as it relates to this thread, is a parser generator you can use to perform markdown-to-HTML conversion (including full HTML inlining), or AsciiDoc/rST or your own custom extension syntax to whatever XMLish angle-bracket markup you wish. So apologies, but considering we have SGML since 1986 or earlier, discussing surface syntax (markdown vs AsciiDoc vs rST vs orgmode or whatever) seems kindof moot and merely a matter of personal preference really.
