Gameplay
Sound - Vehicle warning sounds/beeps are able to be heard both inside the vehicle by not only the driver/pilot but by the passengers/gunner also.
i.e. A transport chopper full off troops is looses it's engines, instead of just the pilot hearing the warning tone all of the troops sitting in the back can hear it also and have time to react appropriately. People should also be able to hear the warning sounds from outside the chopper when it has landed (they would be much softer obviously) and only really noticeable when the rotors are not spinning.
Lighting - Big explosions at night are able to illuminate passing clouds in the night sky (this YouTube link
describes what I'm talking about)It doesn't need to be nearly as bright as this one but it would be cool if it had some effect on the clouds.