I'm a little late in the party but I wanted to express my feelings anyway. I wrote a book with 2 other authors 2 years ago, and would have LOVED a tool like this. Going over Word documents in revision mode was a colorful nightmare. In general, having to work with Word was not the ideal stuff anyway.
To my point: to be totally useful along the whole typical writing process with traditional publishers (authors -> technical review -> authors -> editor review -> authors -> punctuation/misc review -> authors -> editor) you'll need at some point to export as Word documents. Ideally, collaborate in Git like environment, export as Word, receive a new Word with all those colored revision and import into Git like stuff.
To my point: to be totally useful along the whole typical writing process with traditional publishers (authors -> technical review -> authors -> editor review -> authors -> punctuation/misc review -> authors -> editor) you'll need at some point to export as Word documents. Ideally, collaborate in Git like environment, export as Word, receive a new Word with all those colored revision and import into Git like stuff.