Roofs only
This one seems obvious, but it took the death of former President John F. Kennedy for the secret service to change the rules. You are not allowed to drive around with a convertible roof, it leaves you wide open for anyone who would like to take a dangerous shot at you.
So now cars that the President rolls around in are bulletproof and as safe as can be. We have to admit that it must suck that these world leaders are not able to look cool in their fancy convertible cars while they are in office. Must be a drag, always hidden behind bulletproof glass.