Existing repositories with working libraries wouldn't be penalized unless they move to this repository because they would be less "official" than the others ?
My thoughts exactly. I've put my own libraries on bitbucket.org only last week, because I've experience with Mercurial, not with Git. Other people might prefer SVN. Or Bazaar. Or none at all. Can you make sure these are deemed equally "official" as the libraries on the Git repository?
Also, and perhaps I'm not well informed, but I'd like to see a more extended and standardized way to integrate a reference of third-party libraries in the IDEs. I think that's more beneficial for the users of the library than a large GIT repository, which might be quite alien for beginners.
Finally, my project is a package of libraries, which can be used stand-alone but work nicely together. Would such package-construct fit in your ideas of the repository?