Big problem with XM: Not user installable, dealers want you to drop off your car for half a day or schedule two weeks in advance to get it installed.
Big problem with cars: Not enough electrical sockets. I want to be able to plug anything into my car! I don’t want to have to jury rig a connection to the fuse box. The cigarette lighter doesn’t cut it. There aren’t enough of them and they are inelegant.
Big problem with car radios: To hard to hook up arbitrary device to the speakers. I just want to play my audible.com books from my Tungsten Palm, but right now if I wanted to do that I’d have to burn the audio book to half a dozen CD and either play them that way, or then encode the audio CD to mp3 format and put them on a single disk. That is a lot of work, maybe more than listening to the book!