FWIW... I'd like to see this too.
But, I also agree completely with MrC and others above on this:
Genre tagging is very personal, and this means any collective database will generally not be useful for many (is The Cure in genre rock or alternative, is Mark Knopfler rock, pop, country?).
True, I wouldn't actually use whatever [Genre] gets set to by such a system. Genre is a core part of my metadata management system, and I need to manage that manually. But, on import of new artists, it might get me closer. Sometimes, I'm not sure what I'm going to tag something when I import it. I buy tracks and albums on Amazon on whims sometimes, if it gets "recommended" by someone I trust, or goes on sale and has good reviews and seems like it might be something I'd like (this has almost always worked out well, and if it fails, I usually lost less than a cup of coffee and a sandwich). So, how is this not just "better than" what the files come pre-tagged with, or what YADB suggests (which is often uselessly vague)?
But, more importantly, I don't just want [Genre]. I want Bios, and Descriptions, and Release Dates, and Lyrics (I know, probably not lyrics), and locations for live events. Something like this, if it was also able to submit things to YADB (or whatever, accept user-generated content from MC as well) could solve problems like kstuart's and my shn conversion problem for live shows. I think it could be a massive help for Jazz and Classical tagging.
I agree, in particular about [Genre], it would NEVER be right for me, and there would
have to be a way to be selective about it (which tags it uses and which it doesn't use, minimum), but I think it could get you a start, and it would certainly help to fill in a lot of the "more esoteric" tagging data about my files (which are now, only generally filled for stuff I love, and the rest is missing or completely inconsistent).