An experience that will remain with a player, long after the credits roll is, in my opinion, the effectiveness of the story. Truly great games stick with you because of memorable story elements that made you laugh, smile or cry.
I was unconsciously humming a pleasant little tune when a woman in the next line started humming a line of harmony. We laughed. We couldn't figure out where it was from, but a young man who was bagging groceries knew immediately -- it was the theme to "Sonic The Hedgehog."