One of the things I worry about in these discussions is that people are way too quick to treat summary statements on copyright law (like "Copyright protects expression, not ideas" and "Copyright does not protect the idea for a game, its name or title, or the method or methods for playing it") as the end-all-be-all of copyright law.
The problem with them is that the copyright law of "derivative works" is full of vagueness and judgment calls that cannot be easily boiled down. Further, the guidelines used are different for different types of works. A degree of rephrasing and visual distinction that would be perfectly fine if you were making a knockoff of a board game might be insufficient for a video game under the "sequence, structure, and organization" doctrine for software.
What rules apply to a pen-and-paper roleplaying game? As far as I know, there's no on-point precedent, no case law where an RPG was challenged as an infringing derivative work and the suit went all the way to an actual judgment. A good copyright lawyer will be able to give advice, but I expect that a good deal of that advice is going to be "In this instance things are murky, because . . ."
Maybe I worry too much. But then, I thought the everybody (including a number of lawyers) who were all poking and prodding at the OGL back in 2000 had thoroughly probed for and identified all the traps. The last two weeks put paid to that notion.