The Ten Principles for a Good Level Design is one of the most famous talks given at GDC by Dan Taylor and from time to time I like to hear it again while I do some brainless activity around the house.
1- Good level design is fun to navigate. 2- Good level design does not rely on words. 3- Good level design tells what to do but never how to do it. 4- Good level design constantly teaches. 5- Good level design is surprising. 6- Good level design empowers the player. 7- Good level design is easy, medium, and hard. 8- Good level design is efficient. 9- Good level design creates emotion. 10- Good level design is driven by mechanics.