If you use Beyond Compare as the diff tool for your source-control system (As we do, with Perforce), it will show you differences between revisions of your Word docs. Admittedly, it only shows the textual differences - formatting changes are not shown - but this is usually enough for you to see what changed.
This is just another reason to invest in Beyond Compare, as it is one of the most polished pieces of software I've ever used - and it's the best $30 dollars (Less if you buy several) I've spent on software
