The first few words or lines of your game description could be the thing between having people take an interest versus them leaving quickly. If they take an interest it will help you build momentum to finish your project. It might also motivate others to help you on your project. If no one takes an interest it will be a lonely road towards completion. If you do complete on your own you will have missed the potential community excitement that can generate around a game which will help it become a success.
So what's good and what's bad to say? The tone of your description should be 'tempered enthusiasm.' That is where you want to shoot for.
You don't want to come off as overly excited and reflect an immature and 'unlikely to accomplish the task of competition' with comments like 'this is going to be the most awesomeist game ever,' noting that using non-existant words like 'awesomeist' is also a big no-no, while on the other hand you don't want to portray a 'debbie-downer' mood which comes off as if you're already defeated (or humble to an extreme) with statements like 'the models will be low polygon/minimalist' that infer a lack of detail when one could of said 'the models will be rich in detail and optimized for great performance.'
1) Be succinct, but don't just line up a bunch of adjectives one after the other with things that could be better summed up in a screen shot.
2) Present yourself Professionally.
3) Establish an objective mood with 'tempered enthusiasm.'
4) Write not as yourself buy as if someone wrote the description for you. Check out already released game descriptions to get the feel of it. You can add a 'personal paragraph' at the end if you want to connect with other users on a personal level.
5) Publish your description 3 days after you write it. Why? It is important that you reread your description after a few days to make sure you understand what you wrote, so that others can easily understand it. Believe me when I say it is easy to think you are clear with what you write, only to realize later that something you said was actually confusing.
6) Offer a screen shot, logo or something visual even if that means taking a photograph and applying some PhotoShop filtering effect to it. We want to see the mood of the game with our own eyes.
Remember that in the end what needs to be achieved is a professional level of excellence, i.e. no mistakes. This is to elicit a feeling of confidence in the readers minds for what you are trying to accomplish. If you don't sound professional, then we won't expect too much from you.
Best of luck. -bean.