Often, it is best to start with a low-fi mockup to capture the general ideas/vision for a site.  Especially when you have a site with a lot of features/functions (which this one did), it was nice to prototype it first to be able to get feedback about how it all fit together.  By running though a mockup stage, then a prototype stage after it, the project was completed much faster than it would have been if I had just dove into the full design from the get-go.