A list apart has an article on the ways in which increasing form interactivity lowers error rates and improves user experience.
Inspect Element has an article on button design and how it can improve (or detract from) usability.
This article from User Focus depicts a way to communicate user-centered design to executives in just a single page.
All About Balance features an example of confusing UI on a copy machine.
Complete Usability explains how small usability problems can add up to big headaches.
And finally, Useit has a 20-year retrospective on discount usability testing.
