Why your car stereo needs a joystick
I think that user experience is important for software, but also for hardware. Think about it: the more you interact with something, either it becomes more invisible to you (and thus becomes "easy to use") or the more its poor design either bugs you (and you eventually learn to live with it or replace it with something better.)
Shown here is the best interface I've seen for selecting which speakers your car stereo should play through. 
It's made by DaimlerChrysler and is available in Chryslers, Jeeps, and Dodges. I'm not sure if they have a patent on it, but I'm surprised that I haven't seen it in any other cars. In the cars that I drive, I have to push on a knob several times to get to the balance. The mapping here is obvious, in that turning left makes it go to the left and vice-versa. I then have to push again to get to the fade. Here the mapping is somewhat arbitrary. In one car, turning to the left means to the front. However, in the other car, turning to the left means make the sound come out of the rear. Even if you remember the mappings appropriately, you've got to do a lot to make the sound come out of only your own speaker (which is handy on long road trips and you don't want to bother your passengers.)
Contrast this with the DaimlerChrysler "stereo joystick" speaker selector, as I like to call it. You can quickly move the sound to whereever you want it, even to just your speaker in the front-left. The current setting is always visible, whereas with most stereos, you'll have to rely on your ears or press the knob a few times to see the balance and the fade separately. The tactile movement is also more natural, in that you move the joystick around in 2-D vs spinning a knob. Donald Norman, one of my favorite user experience gurus, calls this "natural mapping", in that "the form of the motion would mimic the function", that is moving the joystick to the back makes the sound come out of the back.
I think Donald Norman would be happy, as he writes about this in The Design of Everyday Things. He says that while most of the interfaces in a car are easy to use, there is one that he singles out as being bad: the controls for the speakers in your car. He describes the same problem as that above: even though a knob is simple, which way do you rotate it to get speakers to play from the front or back? He suggests mounting the knob vertically or so it spins forwards and backwards. While these suggestions are good, I think the DaimlerChrysler stereo joystick solution is the best I've seen so far.