I think story. Half Life and Fallout have amazing story lines and that makes them amazing games. Games that have great gameplay but no story line or a bland story can be fun at first, but get old really quickly (Like Gears of War 1. Which is why the other 2 games are better).
That being said if a game has schitt gameplay and a good story (Like Haze for example) the game is crap.
That being said if a game has schitt gameplay and a good story (Like Haze for example) the game is crap.