An open world can really make a game good for me, I stopped playing Final Fantasy XIII just before the end and I really can't get back into it because after putting me through over 20 hours walking in a straight line, then finally opening up the world and letting me free roam and then putting me straight back to walking in a straight line again I just got fed up.
Other than that though a good story, characterization and atmosphere. (Just to use Final Fantasy as an example again) I didn't really consider Final Fantasy VII to have great gameplay, I could breeze through it with the minimum of strategy, but the story made up for that really.