From Social Patterns
Soliciting feedback, in all its forms, from users is one of the easiest ways to engage your users. It has one of the lowest barriers to entry and is often the first step on the ladder of user participation.
User Ratings is potentially the easiest item to add to a site to gather information and start a user down the participation road. Additionally, as you build up the engine around ratings, the information can be used to understand your users and create more value for them through recommendations. Amazon has been quite successful at this, using purchasing behavior and ratings to infer potentially interesting new products to the purchaser.
A step up the ladder is Tagging which involves a little more involvement but still no major commitment by the user.
Finally, leaving comments—which is the first step in having user-to-user conversation about an item (see the Conversation patterns), giving feedback and reviewing an item are all activities that will potentially grab your user for longer term activity. Each of these encourages registration and repeat visitation. These patterns can be combined with one of the rating styles and tagging to create a robust suite of user opinion around your content or user generated content.