I love the concept of branching which really take something great from (D)VCs to apply it to document reviewing. I always had trouble using word's revision system and having multiple people correcting/working on the same document at once. It will highly depend on how you handle the merge and if it's intuitive enough to use.

From you signup process, it seems like you only plan to develop these functionalities. Is it demo-ready yet? Or are these only mockups to ensure which functionality really matters to the users?

EDIT: minor correction, I think visualize is written with a Z but I'm not an english native-speaker so I could be wrong.