Big fan of Draft (and Nate). It's got the version control part down, and I think it works well for gathering feedback on blog posts and other short form content, but it isn't designed for longer form content as far as I can tell. It's also missing the community aspect that GitHub has, which is huge, as well as features that would make writing an ebook or textbook feasible. Definitely a great tool though.

Any reasons not to work with Nate? It seems like the social stuff could be built in tandem with his strong editor.

https://draftin.com/ - Agreed, great.

http://www.roughdraft.io/ - Also, I like it.

I don't think it's a bad idea to proceed with it, I would just find a way to differentiate it from what's out there.

ditto. simple and easy to use.