A comfortable driving position is No. 1 on our list. A great sound system. Wheel-mounted controls for both radio and cruise control. It takes some getting used to changing stations without reaching over to the radio, but you'll realize how often you took your eyes off the road doing it the other way. Outside temperature indicator.
Automatic temperature controls for both passenger and driver.
Drivers often like it a bit cooler on their side to help stay awake on trips, passengers want it warmer to doze off. This is a nice touch, and widely available. Don't leave home without it - the address of your destination, that is. Plug it into the system and never have to argue about stopping to ask directions again. The more the merrier.