BioShock is about the story, the moral choices, the collapse of a once truly utopian society.
It's about the implementation and combination of varied disparate violent elements used for the action, to allow the player to defend themselves through a myriad of unique ways. The combat as customizable as the weapons and self modifications used to wage these battles.
It's about all the little things, the details in the world, it's visual representation and the degradation of that which was once fantasical and pristine.
It's about its darkness and mood of isolation, desperation for escape and survival, and perhaps the yearning to help those not yet completely gone, beyond the players ability to aid them.
It's about Why and How did it all go so very wrong?