The GSL, so far as I know, hasn't been released even to the people who are on WotC's "short list" of third party publishers. Likewise, even if it had been released, its terms are still covered by NDA, so no one outside of those publishers will know much about its restrictions until its public release on June 6, 2008.
So, I don't have much to offer of substance here beyond the usual wild speculations. Here's another one: the GSL -- or, should I say the Dungeons & Dragons 4E Game System License, as it's now called -- won't be viral, by which I mean that it won't allow third party publishers to use the work of other publishers in their own products. What the GSL will turn out to be is a limited use trademark license. WotC will allow third parties to reference 4e's rules, include 4e-derived stat blocks, and use a version of the 4e logo for a limited range of products (certainly adventures, probably campaign settings, and possibly some sub-set of rules expansions), but there will be no "open content" in the sense that the OGL/D20 STL defined it. What this will mean is that each publisher will exist in his own little world -- tied to the rules and branding of 4e but isolated from the products of other publishers.
We already know WotC is keen to ensure that no one produces another Mutants & Masterminds or Spycraft with 4e. Likewise, they've said that they want to make the OGL and the GSL "mutually exclusive" licenses. By making the GSL non-viral they further ensure that even third party-produced mechanics (assuming such things are even allowed) won't spread beyond their original source, thus reducing the likelihood that someone can make a new RPG based on 4e that isn't, ultimately, just a support product for it rather than something independent.
Again, I may be wrong -- I was wrong that there'd even be a GSL, which, to be fair, there technically still isn't -- so take this all with a grain of salt. Still, if I am right, I'd love to see how anyone can claim that 4e is "open."