Apple is a fantastic case study for a topic like this and I would start off by comparing the new Shuffle (3G) versus the prior version.
|Controls||The 2G Shuffle||The 3G Shuffle|
|Primary Input||5 buttons (press or press-hold variations)||3 buttons (click, hold, or multiple click variations)|
|Feedback Method||Visual (LEDs)||Visual (LEDs), Audio|
Scaling the controls to the content is extraordinarily important and the problem here is that for 10 functions, one product uses 5 buttons and the other uses 3. For example, if the door knob of a door is too small, many people can’t grasp it and apply the right amount of torque to open the door. In this case, Apple might have found the ideal physical control system while minimizing physical controls with the 2G. However, they might have mistaken necessity for an opportunity to be too clever since having fewer controls seems to have made some functions more difficult to access. In other words, the density of functionality with one control method should be kept to a minimum.
Recommendation: Merge the audio feedback system in the 3G with the physical controls of the 2G. It keeps the control systems consistent and simple to understand.