"We consciously chose to make the site design appear plain and simple, despite its deep complexity. A white background, blue links, sans-serif font, and largely gray palette all present the site as a straightforward place. The look of the place must never overwhelm the photos themselves. We also tried to create an egalitarian playing field. At a glance, visitors can't differentiate a professional photographer with an enormous lens from an enthusiast just getting started in photography. There is no indication of "quality" apart from the content itself. That also means that it's up to the viewer to decide for themselves which photos they like to look at and explore without prejudice."

--flickr lead George Oates, writing at length  at A List Apart in a marvelous piece about what other developers can learn about building online experiences from flickr. Tons of good observations, but the simplicity rule is always one to highlight, as is giving the viewer the chance to own the experience..