Graphics. It has been proven time and again that a game does not need good graphics to be good. Granted, it's good to have awesome graphics, but if they're missing then who cares? Only graphicswhores would. >_>
If a game is missing a great storyline (provided storylines are a focal point of the genre) it'll be boring and sometimes even painful to go through.
If a game is missing good gameplay, then it's hardly a good game. Gameplay is the most important aspect of a game. It makes or breaks the game. A game can have horrible story, graphics, music but in the end, if the gameplay is good/great, then the game is still enjoyable.