One of the things I enjoy is reading about the licenses that CC has retired. Usually they do great job of explaining why they are retiring the license. Understanding these use cases and their context is a really informative view on society.
One interesting retired license is the Sampling+ License. They did a really good job of explaining why they were retiring the license. One of the interesting exercise they talk about was how they had to go through the machine readable description to describe the license — basically mapping out the assertions.
Sound+ is interesting because it is targeted for sound. It makes me wonder if sound/audio can still be licensed under Creative Commons if it is not protected by copyright.