Maybe provide some minimal compensation to trusted reviewers in the form of product discounts or free stuff – enough to make knowledgable people take their time

Maybe provide some minimal compensation to trusted reviewers in the form of product discounts or free stuff – enough to make knowledgable people take their time to objectively evaluate something they wanted, but not enough to pull in people who are just willing to do anything to get freebies. You could have a reputation system that provided deeper discounts over time to people who were able to write more-useful reviews (as judged by other users).