I'd really like to see MB adopting genres. Even though they are not completely objective, you can usually get reasonable lists if you allow entities to have multiple genres. This is very similar to the folksonomy tagging we have, but it would be nice to be able to tell a difference between random tag and a genre tag.
An advantage of having genres as first-class entities is that we can then add annotations/aliases to them (e.g. http://www.soundunwound.com/genre/psy-trance/16019987?ref=SG), aliases, have ARs to describe the development of genres (e.g. http://techno.org/electronic-music-guide/).
For now I'd be happy to simply link these genres to folksonomy tags, just to have some definition of which tags are genres. If it turns out to be useful, we can think of a different way to link other entities to genres.