Thought this might be an interesting thread to start for people because I am constantly asking myself that very question when ever I work on one. What goes into a design document? If you had to how would you catagorize a design document? Biggest topics I can think of off the top of my head would be. Game World Game Systems Technology Story (possibly go under game world) Anyone care to add how they would catagorize theirs?

There's no hard-and-fast template for a design document. It's not a science.

A good rule of thumb is to put enough into the document so that you could give that document to a team that has no real prior exposure to your concept, and they'd be able to develop it and end up with a product that is a reasonably accurate rendition of what you envisioned.

My first was just Game World, Game Populace, Gameplay. [imo] The game design document should contain all of the 'rules', a high level description of the content, and none or almost none of the technical implimentation of it.

In gamedev's article section, we have a whole category of example design documents and related articles:
Design Document Articles

Also, I myself have written several chapters towards a how-to-design-computer-games book, which includes an outline of types of content which must be created to make a game and a link to one or two different versions of the game design document I've been compiling for my game design project, Xenallure.
Designing - The Game And Its Content by sunandshadow