This will be clarified more fully later in the Style Guide, but since there's been some mounting confusion about what a description is supposed to be, I'll write a couple of sentences about it:
Descriptions are the cornerstone of a game entry. Next to a proper title and basic release information (platform, publisher, year), descriptions are the most important piece of information you can enter about a game. Since one of the goals MobyGames was founded on was accuracy, we really have to insist that descriptions contain at least one sentence that describes the purpose/plot of the game, and another sentence that describes the game's mechanics. This is easy enough, but there are a couple of things you definitely should not do:
Don't describe the game by basing it entirely on comparisons to other existing games. People reading your description may not be familiar with those other games.
Don't put anything opinionated at all into the description. Don't say that it sucks, don't say that it's great. Don't do it even slightly unless you enjoy getting your description edited. Sentences like:
"(name) is a great game that was undervalued by most gamers"
"(name) is an adventure game sporting innovative level design"
"(name) is a Doom clone that adds nothing new to the genre"
...etc. may be completely true and accurate to you, but they are opinions and we can't have opinions in descriptions because opinions are open to interpretation. A description is supposed to be based in fact, and not open to interpretation. (Now, if you feel people should know about what you think is great or terrible about a game, please by all means write a review! You can say anything you want to in a review as long as it won't get someone sued :-)
Don't copy the back of the box or a website as the description. These are marketing rhetoric, and while they are historically significant and also provide a valuable different viewpoint, they are not objective and should be put into the Company Line/Ad Blurb section instead.
Sorry to sound anal, but hey, it's our job to be anal. :-)