The system automatically adjusts the radio volume up or down, depending on the vehicle's speed and the varying amount of road and wind noise in the cabin. A nice feature for those who commute in heavy traffic with lots of speed changes. Toyota, for instance, in models like the MR-2 Spyder and Corolla, places the radio at the very bottom of the dash. Not only does this take your eyes off the road when you operate the radio, but in a manual transmission car the shifter blocks access to the head unit. Look for a radio that is at least halfway up the center stack.
Large buttons are easier to find and use.
While you're at it, try to find a radio with generous spacing between the buttons too. This makes the radio easier and safer to use as well. The head unit in the Ford Focus is a great example of this. A display with large numbers and a clear readout makes for an enjoyable listening experience. Also, try to find a head unit that shows all the pertinent data in one glance.