This recent post by 37signals, Why we skip Photoshop, got me thinking about the most influential web development project I worked on. It was a corporate Web site redesign. The creative director and I simply sketched out on paper three different ways in which the information could be organized. The sketch was just a box in which we arranged blocks of content. Very simply, but it cut to the heart of how the user would interact with the entire site. From those sketches, I was able to create three prototypes (HTML/CSS) which we could all click through and actually experience interacting with the content. This was all done within a week's time. No Photoshop. No rounded corners. No fancy background gradients. Graphic elements were added later on, but technically the prototype went right into production. And guess what? The project went according to plan. No mad dash at the end to re-think, re-do or re-slice anything.
After that project, I finally began to understand 'interactive' design. To this day, only until the entire site has been prototyped do I even begin discussing color, font, images, etc. While those are important in presenting or highlighting functions and content, it doesn't get to the heart of how a user interacts.