RMSE also penalizes for a bad prediction of your 1000th-from-the-top best prediction as much as your top prediction.

Amazon's algorithm is actually quite simple (at least as much as can be extrapolated from the outside, and confirmed by interviews with the people that wrote it), but they do a lot of work to place recommendations in context. I think that's one of the critical places to focus on when looking at raw sales boosts.