The best designers have an accurate mental model of their users. In their mind, they can put their product in the hand of the 30 year old floor manager at the Borders at the mall and predict what will happen with shocking accuracy.

They can predict it because they've watched the floor manager interact with past products in the usability lab dozens of times.

And because they have watched that person dozens of times, they know that even thought they can predict extremely well how that person will react, they know they have to run tests anyway, because the best designers know how often they've been surprised in the past.

The best designers have a great mental model of their users, and know when to use it.

The worst designers just use themselves as a model of their users, and their designs are accordingly screwy.