From The New York Times : If You Liked This, Sure to Love That – Winning the Netflix Prize
…shopping over the Web is not a social experience; there are no clever clerks to ask for advice. What’s more, because they have no real space constraints, online stores like Amazon or iTunes can stock millions of titles, making a stack search essentially impossible. This creates the classic problem of choice: how do you decide among an effectively infinite number of options?
…even though Netflix has a good deal of demographic information about its users, the company does not currently use it much to generate movie recommendations; merely knowing who people are, paradoxically, isn’t very predictive of their movie tastes.