@philandjan - you also might want to try using the
Goodreads metadata download plugin. It will populate your tags column only with only genres.
The Goodreads metadata plugin uses a mapping process which was originally designed to turn "popular shelf" names for a book into genre tags. Recently with Goodreads changes to their website, a lot of the need for this mapping is gone as they now do this themselves. So a book will have a genre displayed of "Science Fiction" on the Goodreads website, rather than "sci-fi", "science-fiction" etc from popular shelves of users.
The mappings between Goodreads shelves and Calibre tags still takes place after their website changes (just with some different mappings). There are still certain shelf names that appear as Genres I want to exclude. However they are far simpler. You can see them in the spoiler on the front page of that plugin thread.
I think perhaps what dwanthny is mentioning above is the changes I have planned for Calibre 0.8. The mappings I mention above are currently all hard-coded into the plugin - so if you preferred some slightly different combinations, tag names etc then you either have to change the code or ask me. I am going to have to rewrite the plugin anyway for Calibre 0.8 because of the new way metadata download will work. As part of that I propose to allow users in the plugin configuration dialog to modify the default set of mappings I provide to add/remove their own. So for instance if you want "Paranormal > Romance" genre from Goodreads to be "Paranomal.Romance" in Calibre for a nested hierarchy of tags you will be able to do that etc.