Why not just keep your files in git? Use a git GUI if the command line is intimidating to writers. That way you don't have to create a brand new writing tool. I keep scrivener files in git, it works great.
I like to code and I like to write, but I'd be much more likely to put a Rails app on Github than, say, a short story on which I'd like to collaborate. It's not that Github can't be used as an effective tool for collaborative writing, but rather, that it isn't really the place for it.
I have a lot better chance of social interaction around code on Github versus writing because the primary social object on Github is code.
There is no question that good writing exists on Github and that it shines for particular forms of writing, but the dearth of writers to coders there seems to signify that a service like Penflip has a market that could be better served.
Research often involves code as well, which would make github more suitable.
That's exactly what Penflip is. All projects are backed by git repositories, so you can clone them and work on them locally just like you would with any git repository. The web interface is really just a GUI for git.