What makes a web site good. You know it when you see it but it is very hard to relate to other people. Jeffery Zeldman has a great piece on web design that everyone should read.

His attempt to define web design: 

Web design is the creation of digital environments that facilitate and encourage human activity; reflect or adapt to individual voices and content; and change gracefully over time while always retaining their identity.

 Understanding web design

Is one of the best I've heard. He also compares web design to typography and architecture with great effectiveness. 

I especially like the part about chaning gracefully over time. This one fact is why content management systems are becoming so important to the web development process.