Less is more

A short, but polishedly fun game experience is better than a long game that's unpleasant to play. Players might quit the game at an early stage if they are bored/frustrated, without ever seeing the next levels you put your hard work on.

Marketing is a part of the jam

An appealing cover art is important to get the other jammers to play your game. So is the game's description. Keep in mind that you should save time for these too, not only for the build of your game.

A player is highly unlikely to play your game if it lacks a description or any visuals in its page. Especially if the game is an executable to be downloaded.

I believe a good game description should tell the genre (if it fits any), objective and controls. (Or maybe a vague, yet intriguing description? It might work when you want to make players curiuos.)

Communication

It should be clear what's the game's objective and how to play. If you didn't have time to place text instructions in-game, the instructions should be in the description at the game page, at least.

I don't believe text instructions are that important in game jams for common controls like WASD movement. But at least game must communicate about when and how to interact with game objects, for example.