What is the games story?
Ideally a game would satisfy some kind of role-play. A dungeon crawler should set a player on a perilous adventure. A game about buying real-estate should give the player some sense of excitement about taking chances with large amounts of money.
Is there a point to it?
A sense of satisfaction finishing the game, being the winner or successfully completing the challenge. In a coop game, the players should feel good about working together.
What are the core mechanics?
The steps that players have to take. Make sure that they are fun and or challenging.
What keeps it fun or stimulating?
Interesting mechanics or a compelling story will keep the game fun right up to the end. It is important to keep the players engaged untill the victory conditions are met.
What makes it replay-able?
This can be a randomness or strategy development factor. Some game mechanics, like cards and dice, make the game play differently each time. Some games have nuances that may not be obvious when first played, so strategy and familiarity with the rules and mechanics will increase a players ability to be effective against others.
What actions get rewarded or discouraged?
Games that neither reward or punish a players actions can be unexciting. Make sure that the risk and rewards are fair, that punishments can be overcome, and do not spoil the fun for the players.
What parts and pieces are needed?
Cards, dice, spinners, custom figures, cubes, boards, etc… the complete list of what the maximum number of players need to successfully play the game. Figure out scalability.
How much are the components?
Reality check, time to consider if it is cost effective. Some parts are fairly pricey to get manufactured, may have to consider other components that could be equally effective for less, and that it does that change the playability.