User Interface Design for Programmers -- Chapter 1: Controlling Your Environment Makes You Happy
http://www.joelonsoftware.com/uibook/chapters/fog0000000057.html
QuoteUI is important because it affects the feelings, the emotions, and the mood of your users. If the UI is wrong and the user feels like they can't control your software, they literally won't be happy and they'll blame it on your software. If the UI is smart and things work the way the user expected them to work, they will be cheerful as they manage to accomplish small goals. A user interface is well-designed when the program behaves exactly how the user thought it would. ...everything else is commentary. All the other rules of good UI design are just corollaries.
...even truer today than it was 5 years ago...