[icosa]

Shannon Appelcline has a great overview of rating, ranking, and other systems in the article series "Collective Choice": http://www.skotos.net/articles/TTnT_/TTnT_178.phtml . The key points are that most people want to be nice so you'll have far more activity in the top half of the ratings (3-5 for 5 stars, 6-10 for 10 stars); explicitly describing what each level means makes people more accurate; and instead of using a simple average you should weight it towards the site average to prevent items with few ratings from being misrepresented (e.g. an item with one 5-star rating is not the best ever).