GFDL is a strong copyleft license: fully viral (any work including work under the GFDL must be GFDL), and fully free (rinse, lather, repeat).
Same with CC-BY-SA
If the issue is invariant sections, why not just make a minor change to the license saying that if a document starts without them, it shall remain without them?
Wikimedia projects are almost all released (with the possible exception of Wikinews, which I am not familiar with the details of) with "no invariant sections" anyway. So this is a moot point.
If the issue is the 7 pages of ugly license attached to works you are selling (that other people actually wrote), just print it on pretty paper. Infuse it with perfume. Put it on microfilm. Whatever.
This is hardly a solution. 7 pages of license, regardless of the "ugliness" of the text is a hamper on a single-page article, or on a single image. That's a lot of baggage for a single image or article. In terms of books it's a much smaller issue, but WMF is less then 5% "books".
Changing the rules in the middle of the game -- even if a bunch of you think it's a peachy idea -- is disrespectful, shameful, and completely discrediting to the idea of contributing free content with the confidence of knowing your content will always be free like you intended it to be.
It's not changing rules. The FSF is looking to add provisions to the next release of the GFDL that would make it compatible with the CC-BY-SA license. Our projects are grandfathered into any change to the GFDL because of the "and later versions" clause that is part of our standard copyright statement. All we are talking about here is making the crazy-cross-licensing issues that our projects deal with every day a non-issue. Tell me that this is a bad thing.
I'm afraid I'm with Greg here: please make sure that those of us who disagree with this have the opportunity to remove our contributions, as well as all derivatives of our contributions.
Again, your works are already released under the GFDL v1.2 "or any later version". The next version of the GFDL, whether it's more compatible with CC-BY-SA or not satisfies this requirement. Migrating to the next GFDL will happen anyway, and if we gain more license compatibility, it's all bonus points.
--Andrew Whitworth