We recently published "How people really use the iPhone", an interesting readout of design issues and recommendations for people designing for the iPhone and beyond. Our presentation on this topic at the iPhone Developer's Summit was called the "best presentation of the conference" by several attendees.
Some of the problem exposed applies to other system imho.
A good UI should fade away, putting content in the front seat — it should be transparent. Sometimes there can be too much ‘UI’ — controls and buttons that are too strong and distracting win over content in their battle for attention.
An interface should be effective, habituating, reliable, efficient, and tested. To the extent that doing so does not conflict with these essentials, an interface should also be attractive.