Other iLife apps like iPhoto already support this functionality, like assigning "vacation" or "mountains" keywords to certain photos.
Is it possible to add tags to songs in iTunes without resorting to the rigid options currently provided (i.e. the 'Comments' or 'Grouping' fields)? See this mock-up for an idea of what this type of tagging would be like:
In the above mock-up, we could assign sub-genres to a song. This would allow us to create smart playlists based on these sub-genres or do whatever else we'd like with them.
Currently, someone would have to use comma-separated values in the 'Comments' or 'Groupings' field to apply this type of sub-genre tagging. It's very inflexible and annoying.
Why doesn't this type of tagging exist? Can it be implemented with plugins?