I am not a lawyer. The following is just some information I have picked up along the way. Please consult a lawyer before considering any action based on the information here. Hopefully, you know better than to regard off the cuff comments on a message board as researched legal advice, but just in case: don't.
That said,
I think this is a bit unfair. Even with the so-called "poison pill", the GSL was a very generous sharing of commercial IP.
The OGL + SRD was an experiment. From the point of view of WotC it must surely count as at best a mixed success.
Actually, the original GSL was incredibly stingy. It was in almost every way worse and more legally risky than using no license at all. The OGL+SRD thing was an experiment but not a risky one. From the beginning, the OGL served the interests of WotC, but not simply in establishing market dominance. The OGL protected WotC by decreasing the chances they would end up in a costly legal battle with a third party publisher.
Let's review a few basics about IP.
1. You can't copyright rules or ideas, only expressions of ideas.
2. You can't copyright or patent prior art or trivial inventions.
3. Not only can you claim compatibility with other people's products, it's restraint of trade for them to unduly interfere with you producing compatible products.
4. Fair use covers a lot of things, including a lot of stuff that many corporations wish or think it doesn't. However, there's a big gray area in implementation, and no one wants to go there, unless they just want to spend lots of money and be worse off than they were. That is why big companies resort to IP fraud; if they can intimidate people into staying away from their properties, they can avoid the costs of actually going to court.
5. You can't copyright simple lists or information.
6. You can't copyright titles.
7. You can't copyright ordinary phrases.
Think about how much that covers in a roleplaying game. As long as you don't plagiarize sentences or paragraphs, there's a lot you can do with that. Many people don't realize this, but Kingdoms of Kalamar began as an unauthorized AD&D campaign sourcebook. Not only did they not get sued, early in the 3e lifecycle Kenzer got a license to produce official D&D Kingdoms of Kalamar.
So, a merely
average GSL would grant at a minimum, free and clear, all the things you can already do without a license. A generous license? We have never seen such a thing from WotC. The GSL was intended for one purpose only: to limit and restrict access to WotC's IP. In exchange for capitulating, WotC agrees not to frivalously sue you. Unless they feel like it, or unless it's Wednesday.