For those that want to contribute here are some more details on what is needed.
Today eZ Publish Platform 5.x and eZ Platform only has a Rating Field type that represent the boolean value of enable/disable rating.
What is missing is to add a Bundle that adds:
1. The missing Rating Service which deals with CRUD operations on ratings and aggregate rating stats per content for use in 2. and 4.
2. Override the field templates to expose rating
3. Criterion, SortClause and implementation in SQL and Solr to be able to filter and sort results by rating
4. Add controller that handles security of it all and business logic of only allowing a user to rate once (but if authenticated user should be able to change rating)