This is by far the most common beginner's mistake, and even experienced designers often stumble over it. I use the term cosmetics to describe those elements of the game that are meant primarily to look or sound good rather than to further the gameplay itself. Another term for the same concept is eye candy.
There are five common motivations to equip a game with good graphics and sound:
To further the gameplay
To permit the player to show off the superior cosmetic capabilities of his new computer
To show off the superior technical prowess of the programmer
To keep up with the competition
To provide the player with images and sounds that are intrinsically pleasing
Reason #1: To further the gameplay by making the player's situation ...