The casual observer might believe that the usability difference between pen and touch is small. But using a pen is an unnatural act, one that until very recently only a tiny minority of people ever engaged in. The psychological payoffs for using a pen on paper are the tactile feel of the paper, the instant feedback of the trail of ink and the physicality of stacks and files and binders of paper notes. Pen-based computer systems don't offer any of those payoffs.

Meanwhile, interacting with and manipulating objects in our environment with our fingers is the most natural of acts. People have been doing it since before our species even existed.

Most users don't really know why they like or don't like different user interfaces. They don't know why tablet-based and stylus-based devices are somewhat irritating to use, and why touch-based interfaces like the iPhone are a joy to use. The reason can be found not in the design of the gadget, but in the design of humans -- we're the ones who are touch-based systems.

In one respect, pen and touch systems are opposites. One makes us adapt, the other adapts to us.

The main difference in usability between yesterday's tablets and next year's tablets is touch. And touch will make all the difference in the world.