The primary one that jumps out is to always use the asynchronous storage device methods. If you use the non-asynchronous one on an Xbox 360 with more than one storage device you will lock it up.
There are things beyond that such as making sure to adhere to the
television safe zones, allowing controllers other than PlayerIndex.One be the primary game pad in your game, and others. There's a whole list of things to think about for XBLCG that you can apply to DBP if you wanted:
http://xnawiki.com/index.php?title=Creating_Great_Community_Games.
As for the accompanying document, the official rules don't really say. You'll probably want to describe your game, how it plays, inspiration for it, and/or why it's just awesome. Since the rules don't really state any hard requirements for the document, I can't imagine there's much way to get it wrong (assuming you turn in something about your game). For my game, my designer drafted up almost a marketing ad to try and sell our game to the judges. Others have taken different routes.