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.