Stuff like modelling the airflow through engines. Our volumetric throttle code calculates power available at a given intake manifold pressure, and means that ambient air pressure due to height above sea level will have an effect on engine performance depending on the setup of a particular car. And where a car has a turbo, it's properly modelled (airflow, spin up, wastegate, blow-off valve) rather than just arbitrarily applying a multiplier to the number labelled 'speed'. And our tyre model is amazing, too.
Jesus.