I think the main thing would be to build a community where people feel some need to belong. I don't know if Amazon really does this, and what you're seeing in t

I think the main thing would be to build a community where people feel some need to belong. I don't know if Amazon really does this, and what you're seeing in terms of number of reviews on their site is probably just another reflection of their popularity. Some % of people will write a review, Amazon just has enough people that it ends up being a rather large number. The problem then becomes how to encourage a greater % of your audience to contribute. Some people might be more willing to contribute if they could see that their contributions were valued. Can users rate a review (like Amazons X of Y people found this review helpful)? Some might be more willing if it increased their standing in the community - some people enjoy being seen as an expert - do you recognize people who take the time to leave reviews (perhaps magnified by review quality)? Most of this will only really work if it is within a community, however. If lots of people just come and go then those tactics are fairly meaningless. If you have a niche site with a lot of repeat visitors that you can build a community around, this might be a good way to start that process.