PPT Slide
The future of user interface design will be idiomatic, where we depend of the natural ability of humans to learn easily and quickly as long as we don't force them to understand how and why.
There is an infinity of idioms waiting to be invented, rather than a limited set of metaphors waiting to be discovered. Metaphors give first-timers a penny's worth of value but cost them dollars worth of problems as they continue to use the software.
It is always better to design idiomatically, only using the occasional metaphor when one falls in our lap.
Alan Cooper: The Myth of metaphor