The goal of this Game Development Competition is more about providing the right kind of motivation to get you to actually sit down and build a game than it is to prove you're a better coder than everyone else. As such, these rules are pretty light, and pretty casual. If one or more rule doesn't suit your needs, feel free to be a Competition Rebel, and have fun anyway.
These rules are tentative. Discussion will follow below, and any changes that are agreed upon will be updated in this post.
0. Winning Conditions: You win if you complete a game in the allotted time. You lose if you do not. (You get a partial victory for creating a partial game.)
1. Create a complete game in the allotted time frame. For Competition #4, this is 31st of January (Saturday) to the 2nd of March(Monday) (31 days total). The words "complete" and "game" are pretty loose. If you think it's complete, it is. If you think it's a game, it is. But it probably means not starting on your game early, and it probably means not having (many) bugs, and it probably means not leaving any half-finished features on the last day. (You can continue to work on the game after.) Starting on the design early, or working on generic cross-game framework code are both allowed.
2. It should be sellable. Not that you have to actually attempt to sell it, and not that if you tried to, somebody would pay money for it, but you should pay careful attention to licensing agreements and avoid copyrighted material (or get permission from the copyright holder). For anything that requires attribution, the copyright holder should be listed in the credits.
The purpose of this is to (a) push people in the direction of making sellable, marketable games and to consider what that might take, and to leave it open as an option, and (b) to make it easier to post images and videos and even sections of your source code to the forums here for bragging/feedback/discussion.
3. Show it off here. To claim victory, you must show off your game here in the forum. A download link is preferable, but screenshots, video captures, and cool descriptions of what you did is sufficient as well.
While not required, consider creating a thread (or more than one) for your game, and give us updates, or get feedback on what others think. Likewise, if somebody is asking for feedback on their game, take some time to give constructive feedback as well. (Please be respectful of the hard work that others have put into their games.)
4. There are no restrictions on team size. If you want to work alone, do it. If you want to work with somebody else that you've found on this forum, do it. If you want to work with some friends, do it. Teams of 1 and teams of 100 are accepted.
5. There are no restrictions on what engines, frameworks, or platforms you use. I suspect most people will use XNA or MonoGame considering that's been the focus of this site, but if you prefer Unity, JMonkeyEngine, or something else, feel free to do so.
"May the mercy of His Divine Shadow fall upon you." - Stanley H. Tweedle, Security Guard class IV, The League of 20,000 planets