@SleepSeeker - there is a recent discussion on tags downloading you might find useful
here.
As per that thread you may find it helpful for your purposes to use a different plugin for the downloading of tags. Below is a screenshot of the configuration screen for the new version of the Goodreads metadata download plugin for 0.8. It is ready for testing but relies on some unreleased changes Kovid has made so you need to be running from source. If anyone wants to try it drop me a PM with your email, otherwise I will release it in the plugin forums for people to try when Calibre 0.7.57 comes out.
As Calibre 0.8 has a feature to limit the number of tags downloaded, this will apply to genres from this plugin as well. Each book on Goodreads gets up to 8 genres displayed for it, ranked by the number of users who have classified them that particular way. So you could set a limit of just 1-2 if you wanted and get the most popular specific values.
In the meantime you can get the same genre mappings when running in "non 0.8 mode" with the existing Goodreads metadata plugin. This new "Goodreads" plugin will replace both "Goodreads Metadata" and "Goodreds Covers".