I would tend towards something light like Markdown, but the needed capabilities depend on what requirements you have for the output (formatting and styling).
I find the AsciiDoc project quite interesting. From their website:
AsciiDoc is a text document format for writing notes, documentation, articles, books, ebooks, slideshows, web pages, blogs and UNIX man pages. AsciiDoc files can be translated to many formats including HTML, PDF, EPUB, man page. AsciiDoc is highly configurable: both the AsciiDoc source file syntax and the backend output markups (which can be almost any type of SGML/XML markup) can be customized and extended by the user.
It even comes with a Vim syntax.