the frontend has to be designed thinking in the person who will use it is driving a car!, so you should be very simple, not distracting, intuitive, able to handle basic functions pause, seek, next song, answer phone call with only three or four buttons. (like stereos do)
The sound is very important, the software should give many audio settings, trebble bass equalizer, loudness, automatic gain control (songs recorded too loud).
And one of most important things is that it is reliable! that no hanging, it is very frustrating that the program will hang or crash when you are driven, or when it's going to power on or off.