It forces me to focus
Working with small screens and touch (vs. mouse) interfaces makes you strip away the extras that clutter many web sites and zero in on the essential experience.
It opens a world of new “contexts” for usage
At the same time as you deal with limits on screen and finger size, you are freed from the context of “I’m sitting down somewhere with a keyboard and a screen in front of me.”
Location and orientation awareness
Oh, the cool things you can do. Like swiveling in your office chair, shooting tie fighters as they zoom past your co-workers.
Steve Jobs has talked enough about this already, but being able to directly manipulate stuff on screen with your fingers is a dramatically different experience than clicking with a mouse. The difference between a modern multi-touch screen and the sort of ATM touch screens we’ve all used is like night and day.