But Genres already are, effectively, tags. Depending on implementation, artists, albums or songs are given one or many genres.
If by tags, you mean user-driven categorization by arbitrary text, that's an even harder problem. Tags can be spelled differently and mean the same thing. Or they can be spelled the same and mean completely different things. What you mean by "punk" and what I mean by "punk" are different. Possibly radically different. If tags are to be useful at all, you still need one authority who decides specifically what each tag means.
If by tags, you mean user-driven categorization by arbitrary text, that's an even harder problem. Tags can be spelled differently and mean the same thing. Or they can be spelled the same and mean completely different things. What you mean by "punk" and what I mean by "punk" are different. Possibly radically different. If tags are to be useful at all, you still need one authority who decides specifically what each tag means.