I've enjoyed reading the
philosophical exhibitionism going on in the Amazon Reviewers
post from last Tuesday. In trying to find latent motivations for the 1,282nd book review on Amazon, I missed a bunch of overt motivations and they've been covered well in the comments. The most obvious motivation is knowledge sharing, as in, "hey, this cajun cookbook doesn't have a recipe for gumbo!" I still don't think this adequately explains thousands of reviews for a single product, because not every review is adding a new data point about the product.