Replay Value (game isn't any good if you don't want to keep playing it)
Gameplay (controls, and generally what you do in game)
Atmosphere (graphical realism isn't important, but the graphics/sound/presentation have to create a unique atmosphere and feeling that heightens the experience and is compelling to be in)
At first I was going to put "Story" into the three, but then I thought about games like L4D which has nearly no story yet is a fantastic game in its own right. Similarly CoD4:Modern Warfare is a fantastic game, and one of the best ever IMO, and its story while good in its own right won't win any awards for total originality. It's a typical Clancy-esque style terrorists with nuclear weapons that elevates itself in the notable "Nuke" sequence.