I’ve been an advocate and heavy user of AsciiDoc (wikipedia.org) for about a decade.
This whole website is generated from AsciiDoc files and it’s also my "native" wiki format in Vim using my vviki plugin!
But I’ve found some rough edges in AsciiDoc and its tooling that I probably can’t live with too much longer for something I use so intimately.
The syntax can be challenging to parse (100% correctly), which often makes things like syntax highlighting unbearably slow for big documents. The final straw for me was when the default Vim AsciiDoc syntax highlighter darn near crashed while trying to understand an (admittedly complicated) URL. That’s just not acceptable. (But having written my own minimal highlighter for Vim for just a tiny portion of AsciiDoc syntax, I understand why!)
Suffice it to say that I’m currently cultivating ideas for a new markup language of my own. (See the list of bullet points at the top of the page to see where AsciiDoc specifically falls short for me right now.)