I dunno. This is pretty bad. While it doesn't explicitly state that the only thing you're allowed to build is modules, this line is the key:
And it's not well defined. Certainly any type of house rule seems prohibited, so Stalker0 is technically not allowed to distribute his wonderful solution to the horribly broken Skill Challenge system.
But it also looks like this line kills any sort of campaign setting uniqueness. I don't want Star Pact Warlocks in the campaign? Too bad, that's altering the definition of Warlock. Healing magic is weaker, so all healing does -1 point? Not allowed. I want Raise Dead to be more difficult, and not available until higher levels, or to not have any cost, but rather a quest component? Not allowed. Want Eladrin to lose their teleportation ability in exchange for something else because the Feywild was destroyed in that campaign world? Not allowed.
You know, in retrospect, maybe it's a good thing there's not much fluff in the books, particularly the Monster Manual, because I don't think the GSL allows deviation from even that.
Plus, there's the fact that if WotC produces anything with the same name as a new class or feat you've created, you potentially have to stop sales of your product and choose a different name, reprinting the books, in hopes that the new one doesn't get yoinked. Great.
So, in short, it looks like the GSL=Modules Only. That's extremely annoying.