I'm not worried about reprints, and I know the laws about copyright and expression-vs-ideas. My point is . . . well. Let's paint a scenario.
Let's say you released a free adventure under the GSL, as your sole product ever under the GSL. Two years from now, the GSL is terminated for everybody by Wizards. You try to release a "Kingdoms of Kalamar"-type product for 4th Edition, unrelated to thast one GSL adventure. Assuming that "Kingdoms of Kalamar" is absolutely, 100% legal, how does the surviving-termination 10.1 affect your rights to produce that sort of thing?
You've agreed, see, that "Licensee understands and agrees that it is not authorized to, and will not utilize, any Wizards Intellectual Property (other than Licensed Materials), including without limitation any trademarks owned by Wizards, except and unless Licensee has entered into a separate licensing agreement with Wizards authorizing such use." And you've agreed not just that "Wizards Intellectual Property" includes "any patent, copyright, trademark, trade dress, trade name or trade secret right and any other intellectual property or proprietary right owned by Wizards", but also that "Wizards Intellectual Property includes all content contained within the Core Rulebooks".
Now, ideas are not subject to copyright. But the ideas expressed in the Core Rulebooks are arguably part of the "content" of the books, which would make them part of "Wizards Intellectual Property" for purposes of the contract, though not for purposes of general law. This is emphasized by Section 18, where we see "concepts, storylines, or other content", which gramatically includes ideas (concepts and storylines) as a type of content. The way a term is used in one part of the contract can often be a guide for interpretation of the term in other parts.
I think it's reasonably certain that 10.1 tries (how successfully it's hard to say as a non-lawyer) to make sure you sign away nominative use of Wizards trademarks and fair use of Wizards-copyrighted material. What I think is possible is that it also tries to make sure that, if you ever do anything under the GSL, from then on Wizards can sue you for breach of contract if you ever produce any 4e-compatible material without using the GSL.
(Yeah, this is both abtruse and getting away from the thread, since it's not really a showstopper, just an added little spike.)