Apple is now requiring that users who wish to review an App Store application actually have downloaded the application. Clicking on the "Write a Review" link on an application's App Store page now generates a dialog box if the user does not own the application.
Users have complained in the past that the App Store reviews have been riddled with poor reviews and comments from users who have never used the applications, so limiting reviews to users who own the applications should provide more reliable and informative reviews.