Sound card with 4.1 RCA 6V outputs, shielded from the rest of the pc.
2 headphone out, and 2 VGA out for the kids on the backseat, so I can see and hear GPS,
while kids are watching Shrek n+1
(I don't have kids, but I'm planning an install for one of my friends too)
Cheap capacitive touchscreen, with LED backlight TFT (TFT is cheap, LEDs are cheap,
I bought a 9" capacitive touchscreen for ~$40, so what costs $700 in a 7" screen?)
I think Bluetooth, GPS, FM, WiFi, GPRS are better when external USB, for many reasons.
You can power on an USB device anytime when you need it, but if you disabled Bluetooth
in the BIOS, it's a bit difficult to turn it on without restart. With too many integrated devices,
the boot time will be slow as hell.
Integrated wifi (like on the Zotac board) has no point in a carpc, on the rare occasions when
you want to use it, you can just connect an usb wifi dongle.