I've been looking at some of the tag editors out there, the tagging capabilities supported by oggtags, apetags, ID3v2-4, and code to support them. And I found a list of suggested tags by a "community of Ogg/Vorbis users for their own use."
The list of suggested oggtags was drawn up to meet specific, reasonable, goals. And one of the stated non-goals was stuffing graphics, liner notes and pet names in the file. But it's still pretty extensive, and I just can't see your average user entering alla that stuff. Well, I can't see ME doing it anyway. Personally, I have very modest tagging needs--what's the track's name, who performed it, where I got it from, maybe the genre (knowing who performed it generally tells me that).
So I'm wondering what most folks, and most audiophiles, feel is the minimum required info an audio file should carry around with it.