I love Sanderson's books, but I find his exploration of the medium of fantasy writing a whole lot more fascinating.
Sanderson’s First Law of Magics: An author’s ability to solve conflict with magic is DIRECTLY PROPORTIONAL to how well the reader understands said magic
Sanderson’s Second Law can be written very simply. It goes like this: Limitations > Powers. (Or, if you want to write it in clever electrical notation, you could say it this way: Ω > | though that would probably drive a scientist crazy.)
The third law is as follows: Expand what you already have before you add something new.