I was reminded by the recent release of the new Affero GPL (very welcome news!) of the ongoing revision of another Free Software Foundation license, the GNU Free Documentation License (and its derivative, the Simpler FDL).
The GFDL is the license used by Wikipedia, resulting in a wealth of free content available under that license. Unfortunately, its copyleft clause only allows combinations with GFDL-licensed works — there’s no compatibility with functionally equivalent Creative Commons licenses, i.e. BY-SA.
For years, there’s been talk from all sides on working toward interoperability. But it hasn’t happened yet. I’m holding out hope for a breakthrough in the GFDL revision process.
If you want to participate, you can comment on the GFDL draft. Asheesh Laroia has posted a comment encouraging work toward a solution. After logging in, you can “agree” with the comment (here) or add your own comments in reply.