The GDD, a daunting, boring, tedious task that is nothing but an outline of all the tasks to come. I've had this conversation a lot, and it usually starts with someone saying, "Why even make a GDD?" They have a lot of reasons behind the question, but they're usually something like these: "I know what the game needs, The game is going to change during development anyway, Doing this will just confine me," and others!
So why even write one?
Without rambling, it is my firm belief that you absolutely, positively need a Game Design Document. Once you have the solid foundations of your idea and you think you're ready to start the task of building your game, you need to force yourself to sit down and think about every single detail that will go into making your game.
The two biggest advantages of doing so are that you will be able to make more realistic goals for yourself once you start writing down and seeing just how much freakin' work goes into making a game, and you'll be able to flesh out your game features better.
For the first, this could mean allowing you the opportunity that you shouldn't do this alone. Getting a buddy to work with is not a bad idea, so long as you can trust his or her devotion to the project.
It could also mean on cutting back on some of the more complex features in your game, or perhaps reducing the amount of variation you originally planned for enemies and terrain. Remember that art assets are time-consuming at least, and budget-consuming at worst.
The second benefit is one that some indie designers tend to scoff at. Why write everything down? The game will evolve naturally during progress!
That may be true, at least partially. But it will also evolve wildly. Making up new features as you go can be dangerous, and runs a strong risk of making you eventually lose control of how much work your game needs. You'll start with the idea that you'll have three dungeons and a boss in each, and then three months down the road you realize that the second dungeon is too repetitive so you start adding a secondary branch to the dungeon with a mini boss and a fleet of unicorn ships. Or something.
|Apparently Unicorn Ships are a real thing.|
So I promised not to ramble, but I guess that didn't work. Anyway, the point is: WRITE THE GDD! It will save you some headaches and make other people take you a lot more seriously!