Game Design Documents (GDDs) are the bread-and-butter of a Game Designer's role, and just like bread (more so than butter), they come in many different types and textures. Some designers believe in a 200-page Game Design Bible, which details everything about the game, and some believe in separate documents to be used for different purposes; in short, different designers tend to write the GDD in different styles based on the dynamics of the development team.
I personally believe in the David Jaffe school of GDDs, which generally follows the KISS rule, "Keep it simple, stupid." My general rule of thumb is not to include any unnecessary information in the document just for the sake of it, as many people are tempted to do for the sake of convenience. As with any form of communication, the most important thing is getting the message across to the people that matter, through whatever means possible.
Game Design Documents
Below are some samples of the GDDs that I have written. Meteor Garden's GDD is a more for general use and to convey the game concept, whereas Mūzaïc's GDD was one written more for internal use to convey detail the design to the rest of the development team.