Alongside the FDL the FSF have also proposed a Simpler Free Documentation Licence (SFDL). This is a great improvement on the FDL as it does not allow invariant sections.
Invariant Sections are the moral and practical equivalent of Adware. They allow advertising, obnoxious commentary and other irrelevences to be added to a document in such a way that you cannot remove them. When you share or modify a work these sections you must pay the price of imposing these sections on others if you wish to use the original work.
This is unacceptable in a Free licence, and is largely the product of the FSF’s desire to propagate the GNU Manifesto. More people should read the GNU manifesto, but not at the price that the FDL extracts. The answer to the FSF wanting to add the GNU Manifesto to their documentation should be “sure, just don’t make it invariant”, not “sure, as long as everybody else can add invariant adware as well.
If BY-SA was made compatible with the FDL, Invariant sections could be imposed on derivatives of BY-SA works. This would be unfair on BY-SA licensors, and might effectively alienate them from otherwise usable derivatives of their work.
The SFDL imposes no such price on derivatives and so BY-SA/SFDL compatibility is not problematic.
The SFDL is also compatible with FDL work that has not invariant sections (but not vice versa), making it compatible with Wikipedia. If Wikipedia went to SFDL then it could be BY-SA compatible.
For these reason, when commenting on the FDL revision web site, discussing this on mailing lists, or blogging about it, please call for BY-SA/SFDL compatibility, not BY-SA/FDL compatibility.
And if you believe that Invariant Sections are bad (which they are), ask the FSF to deprecate the FDL in favour of the SFDL. Eben Moglen did say that Invariant Sections would be gone at one stage, the community should push for this again.