This is a case where a bit of skeuomorphism (gasp) might have been useful. The momentary screen refresh flash on old-style LCD calculators helped indicate to the user that the input had been acknowledged. In the iOS calculator, the subtle darkening of the "=" button (which is also almost entirely concealed by the rest of the curled thumb in right-handed users) is just not enough feedback on its own.
I don't think you mean skeuomorphism, but the point is valid. There should be some sort of action indication. It's really pretty bad design.