They seem to have done extensive usability testing based on a user-centred design approach:
We consulted the foremost experts in the field: our customers.
We traveled around the world, inviting customers like you to come and try out the new features and design. We listened to their feedback and made changes based on their opinions. Then we asked more customers for their advice, and we made more changes from their feedback. The design you see today reflects the input of many real-life customers of our U.S. and international websites.
We concentrated on shopping, searching, saving, and buying–the four activities that customers have repeatedly told us are the most important to them. They’re now prominently featured at the top of every page on the site.
(via Experience Solutions)