Beyond Compare 4 is able to diff text (read-only) from two docx files, under Win/Lin/Mac without Office program installed, which is very handy an quick.
But at my workplace we only own BC3, and at home I stick with SmartGit Built-in Diff Tool.
I believe this could be easily advertised as a big feature for companies that likes to put documentation into source control.
Other format like odf could be useful for other people.
Customer support service by UserEcho