I think the proper way to add rating support to zotero is to move the tag system to a full taxonomy system just as drupal and wordpress did. Then It will allow users and developers to add rating features, tags, categories, locations, school of thought, etc.
You can take a look how wordpress handled it: http://codex.wordpress.org/WordPress_Taxonomy
taxonomies do not need to be hierarchical.
User case example
For a single user:
items could be assigned to categories: (ex: Economy, macroeconomics…)
items could be tagged (ex: international trade, balance of payments, free trade)
items could be located (ex: France, Germany)
items could be rated by usefulness (ex: 4/5)
items could be rated by likeness (ex: 2/5)
For a social environment all the terms could be treated as tags.
in the example above: Economy, macroeconomics, international trade, balance of payments, free trade, France, Germany, usefulness:4/5, likeness:2/5
We might loose some of the meaning but aren’t tags in somne way subjective after all and from my point of view we would gain more flexibility when sorting/searching, as every user would be able to decide how to sort things out: categories and tags, only tags, tags and rating, places and time, etc.