Go read everything Dan Cook has ever written. Seriously.
But start with The Chemistry of Game Design, because I know you don't have much time.
In a nutshell, Danc talks about what he calls skill atoms. These are the tiniest differentiated skills that you can classify in game design. Essentially, these can be constructed into a map of the skills the player needs to learn to play a game.
This is a good idea because it allows a designer to understand where players are getting lost, and not mastering skills. A good (commercial) game design is one where everyone has a fair shot at completing the game. We as designers want players to be able to play. Introducing skills that are hard to understand or master may be considered a worthy challenge for some players, but most will give up and throw a controller in frustration.
"Overall, the skill atoms brought a large amount of clarity to the process of critiquing the prototype. The ‘typical’ method of ad hoc brainstorming led to considerable thought, but very little concrete communication. This has been a major problem with game design conversations for decades and it was delightful to see communication issues emerge so clearly in our prototyping exercise." - Project Horseshoe 08