First: I think this thread should be moved to "Gameplay & Game Design" (maybe?)

Next:
What makes a good design document?
Well I think it depends on what you aim for.
When you want to do a rather big and complex game with a team you need a really good design document explaining every detail of the game. (look & feel, gameplay/game mechanics, characters, story, locations, mission and so on)
You would have to write it 100% detailed so that everyone reading it would exactly know how this will look like.

But that is maybe not so important for a small game, which you are creating on your own.
Then you may put your attention more on planned features, etc.
It would be more of a guide and to-do-list for you (the only developer) and some help to stay focused on what you planned.
However you should always describe what the game is about etc. to get back to it more easily when you had to put it on ice for some time (for whatever reason).

Just my two cents