For UI guidance, don't look at existing CarPC software, or desktop software, or media center software - look at something that uses a touch screen. The iPhone is an obvious example, but there are others.
The most important thing to is to design the UI around a 7" touch screen with 800x640 or 800x480 resolution. Design it with the assumption that the user would rather be looking at the road than at the screen. Then test the UI with that hardware as often as possible. If you do 99% of the development on a desktop computer it will be far too tempting and too easy to put a dozen buttons on every screen that need hunt-and-peck while driving. Or a dozen little icons/glyphs/indicators that the driver will have to squint at to find the one they're interested in.
A UI that works great on 17"+ screen with a mouse is probably going to be awkward in the car on a 7" screen with no mouse and no keyboard. A UI that works great in the car is probably going to be awkward with a mouse. Look for thinks that create those differences, and make something that rocks in the car. Doesn't matter if it sucks with a mouse.
Most of today's front ends don't get this right, and that's the main reason I'm still using my car's own stereo for music. Some front ends get bits of it right, but other parts still have the legacy desktop UI design, and I'm still looking for something satisfying.
Features, in order of preference:
* able to launch 3rd-party applications (navigation especially, but I also have a Subaru-specific engine monitoring app, and proprietary stuff like that will always be far better and far faster than OBD2)
* music library and player
* acts as a hands-free device for bluetooth phones
Nothing else matters to me.