GitHub is closed in the sense that you can't access most of its source code but it's not really a problem since Git is decentralised. When you clone a repository from GitHub, you have everything you need to host that repository somewhere else. GitHub doesn't take any power away from the project - it only provides tools to easily manage your project.

I'm not sure that status of downloading a project's issues or wiki but I remember they were working on making them clonable.

I think the value of the community generally outweighs any concerns about them not releasing the source for it.

That said, I would love an open-source equivalent of Github. It would be fantastic for projects to be able to create an equivalent for members of their project and authors of the plugins, skins and other add-ons that surround it.