Designing a Graphical User Interface for today's applications means making the most out of scarce screen real estate. And while a UI could be created without the use of any icons, it is these little pictures that provide a more enjoyable and understandable experience to the user. 

Designing effective icons involves understanding of palette and size limitations within the system where the icon will appear. The symbol must be stripped of any unnecessary information to provide for easy first time use and long term retention. While the ultimate goal of an icon design is to be able to stand alone and be well understood, icons must play together with others in the same set to achieve visual appeal and unity.

Apple's sneak peek at the OS X interface would have us all believe that large, pictorial icons are the wave of the future. That with high resolution displays we can be rid of those tiny boogers adorning toolbar after toolbar of 16 x 16 pixel icons, and have in their stead full color, large pictures to represent things. There is a naive assumption there that bigger and more detailed looking necessarily means more usable. On the contrary, I predict a future where concise visual statements that cut through the perceptual noise we all face will be the preferred approach.