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It would be great to have the genres ordered in a tree-structure.
For example:
-Classical
--New Music
---Atonality
----Electronic Atonality
-Rock
--Hardrock
---Metal
----Black Metal
--Rock'n'Roll
-Electronic
--Techno
--House
--Electronic Atonality (you see the clue: a genre can have multiple parent-genres)
...

I hope you know what I mean. It is sometimes hard to choose categories which are not to small and not to big. Such user-defined structures would make it easier.