I like a nice large font. 'Car' is a bit of overkill, and kind of ugly, so I guess I like a clean, somewhat pretty WPS.
Large, clear font.
Clock & battery displayed somewhere.
Song/Artist/Album displayed in that order.
Countdown & Total track length are nice
Next track is also nice when I'm in the car with my fiancee...she doesn't like all my music so it's nice to know when I'll have to skip a track.
Play/pause icons (no forward/reverse icons...obviously I know if I'm speeding through something).
Play mode indicators (random/repeat/shuffle).
Dark colors.
Right now I'm using a tweaked version of Cabbie 3 with a larger font size, and I like it quite a bit. I've also enjoyed Ultima and Clean Dark.