The first thing to realize is that a Web site's competitive analysis is usually performed for a team of business specialists who know nothing about design, usability, or information architecture.

They don't have a clue about labeling systems, search ergonomics, or affordance. All they want to know is what the competition is doing and how they can do it better. Obviously, your expertise is in usability and user experience design, so you'll be evaluating sites along the lines of your domain expertise, but the data you gather must always point toward making a smart business decision.