A very interesting article! I've never heard of "possibility space" before but i'm kinda familiar with the ways to control, to manipulate (to an extend) the player's behavior and it's always nice to gain some knowledge on the subject. You explained it well and backed it up with world known exemples to make it clearer. I once heard that the best way to test your comprehension of a subject is to teach that subject to someone who's unfamiliar to it. You prove that point!