What's depressing to me -- and my depression is not alleviated by reading Skriv's syntax -- is how similar they are and yet how none of them deal with the many kinds of typographical formatting that are routine in printed books.
For one simple example: Poetry. (Case in point: [1]) People who publish poetry expect as a matter of course to control their line-breaks and their indention in support of their meaning, and to center lines of text. Only AsciiDoc ("verse" block) and Skriv support literal line breaks (outside of monospaced code blocks). I don't know of any of the ascii markups that support centered text, or controlled indents.
Another simple example: Drama, play scripts. (e.g., [2]). Act and Scene titles are centered; character names are often small-cap (try commanding small-cap in any ascii markup); in older published plays it is common to have entrances and exits right-aligned: "[Exeunt" in the right margin. Both verse and drama (not to mention legal documents) often have reference line-numbers in the right margin, on the same line as left-justified text.
Quoted correspondence often has dates and signatures right-aligned. AsciiDoc supports right-alignment only in the single case of the citation of a block quote. I don't know any other text markup that allows even that much.
In short, the couple of dozen extant markups are solving the same limited range of problems over and over for no better reason than general NIH, while ignoring the much wider gamut of published formatting.
[1] http://carl-sandburg.com/chicago.htm
[2] http://www.shakespeare-online.com/plays/macbeth_3_1.html and this is a wretchedly ugly example but look at the source! It's a TABLE!