In his latest post, Greg talks about the "Customers who bought this also bought" feature, internally known at Amazon at the time as the 'similarities' feature. One of the early problems with 'similarities' was the 'Harry Potter' problem.
See, everyone and their dog bought Harry Potter:
"This kind of similarity is not very useful. If I'm looking at the book "The Psychology of Computer Programming", telling me that customers are also interested in Harry Potter is not helpful. Recommending "Peopleware" and "The Mythical Man Month", that is pretty helpful."
This is in the same vein as another of my favorites, the "Bud Vase Feature."