Simple is one-click checkout. Less screens = less clicks = less confusion = more orders.
Simple is limited choices. Think retail models like Costco, that have sufficient variety but limit the number of manufacturers. Think the Google home page. They’ve left it simple when they could fill it with so much.
Simple is anonymous checkout. C’mon retailers, you have everything you need in a purchase to build a customer profile: Name, address, email, purchased products. Set password as an optional field for those visitors that want additional features such as order history, ship status, etc.