[allpratik]

That's so true. And it's not just the relevance part of it, we have to even incorporate few operational cues as well while recommending products. Not for Amazon but for a different marketplace, I manage the recommendation engine as a product and we have incorporated product serviceability to customer's address as one of the parameters. Then the usual e-comm stuff kicks in like, out of stock issues, price elasticity during sale periods, new product/category launches etc.

Obviously, I do not want to discount other non-commerce recommendation engines like Spotfiy's or Netflix. They are great in their own ways, but e-comm is quite a different ball game. The cross sell model is really hard to crack if you have 1000s of competing sub categories and millions of products in them to fill in your top 10-20 slots.