SCREEN SIZE (I like my 10.4" and its daylight readable even in direct sunlight)
Expandability (see next few items)
Storage (I currently have 120G onboard and 1Tb in trunk)
Integration (GPS location based weather: I have 30+ weather shots - 8 of which are animated and updated every minute; GPS based gas prices; GPS based movietimes; front and rear cam loop recording; TPMS/OBD/Fusion Brain interfacing car systems; Phone (I have full control of everything on my phone);Digital camera and other USB connectivity on the fly; etc, etc, etc.
PC Apps: Record FM, TV, XM , and Internet radio to "ARTIST- SONGTITLE.MP3" in music Library; Burn CD/DVD; Readable Email and Web browsing; Video Conferencing; etc, etc, etc.
A CarPC will never replace a smartphone, but my CarPC IS my car phone. This is a hobby - and I've accepted I'll never be done. Its all about the time you have to invest, your priorities, and tastes. Its not for everyone.