Matt Gemmell wrote yet another brilliant post on what makes a great application interface (with examples) in Augmented Paper:

For me, software experiences that feel like Augmented Paper are those that second-guess our (developers’) natural tendency to put functionality first, or to think of our apps as software. Apps are only incidentally software; software is an implementation detail. Instead, apps are experiences.

Honestly, if you are an app designer, this is a must-read.