You might be interested in pandoc: it allows you to write your documents in LaTeX or Markdown and then it can automatically convert it to docx, odt, html, LaTeX, Markdown, and more. For straigtforward[0] documents this works great.
Interesting, thanks for pointing out the updated version of what "straightforward" means. Last time I looked at Markdown you couldn't do a lot of things I needed in it, such as figures, footnotes, and references, so I didn't give it very serious consideration. But it looks like pandoc has extended the original Markdown into some kind of Markdown++, which covers just about everything I usually need.
[0] documents containing text, headings, notes, figures, tables, code listings, mathematics, hyperlinks, lists, references.