In the same way
GET http://musicbrainz.org/ws/1/collection/
returns a list of the user's collection entries,
GET http://musicbrainz.org/ws/1/rating/?type=xml&entity=<entity-type>
should return lists of the user's rated entities in a format like this

<?xml version="1.0" encoding="UTF-8"?>
<metadata xmlns="http://musicbrainz.org/ns/mmd-1.0#">
<release-list count="3">
<release id="cac64a87-42f9-4c1c-a5ef-1e6824e20678" rating="1" />
<release id="b71926ec-5813-4fa4-9372-0c07154a07af" rating="5" />
<release id="172ddda3-1837-4fd2-8d12-ddd1e70b4c57" rating="3" />
</release-list>
</metadata>

Currently the only way to get a list of the user's ratings seems to be by scraping the website's HTML...

A similar interface for the user's tags would be helpful, too: a request to return a list of the names of the user's tags and a request to return a list of the entities tagged with a specific tag.

(this is my first musicbrainz bug, sorry if I submitted it wrongly)