There are many issues with tags:

- multi language - id3v2 tags support having multiple title tags. Yet software will assume there is only one, and won't show alternative titles. This becomes problem when exactly same song has multiple names, one in original script, one transliterated, one English translation.

- old international tags - most software can't deal with Russian KOI8 and Polish Mazovia at the same time

- multiple artists - there are songs that are collaborations of multiple artists. For example: 'Will.I.Am & Brittney Spears' gets treated as own artist, as opposed to collaboration. Music Brainz and Discogs handles this in their data.

- alternative artist names - Some artists produce under alternative names. Most tags assume that 'Norman Cook' and 'FatBoy Slim' are two different artists. Same with 'Richard James' and 'Aphex Twin'. Discogs handles this

- hierarchical, multiple genre tags - Most software supports simple genre tags. Few applications can handle multiple genre tags. What about tag hierarchy (ie: Rock -> Symphonic Rock -> Kraut Rock?).

- Extended info tags: mood, style, bpm, key - Sometimes when I am melancholy I care for love songs, I don't care for genre, it can be jazz, or death metal (last.fm data contains enough to map mood/style/decades). When I DJ, I need info on BPM and Key (ie: A Minor), so that one song fits the next. (from Echonest / Beatport support this)

- composer vs performer - for classical music it is as important to know who composed a piece (ie: Chopin - Funeral March) as who was the perfomer (was it Rubenstein or Padarewski)

- spoken word - music listening is quite different from spoken word, like audiobooks. Most software sees just a music file and doesn't handle proper spoken word distinction, while use case is very different

- tag sources - while writting tags, there should be additional tag saying where it came from. Foobar's Foo_discogs and Foo_musicBrainz plugins will write source ID, so that at a later time songs can be updated to online db (assuming discogs/musicbrains has updated their db entry)

- song popularity - there are tags to track song popularity and ratings, yet most software ignores it, or does this badly themselves

- stand alone songs vs mixed songs - some albums / artists have stand alone songs. These can be listened to randomly. Other albums will have songs flow one into another (listen to any Jean Michelle Jarre album, or live dance music split into tracks). There is no proper handling of how these flow

- replay gain - most software is not smart to handle album vs song replay gain properly. Not to mention that there are two types of replay gain for mp3s alone. One will change quantization values, one will write tags.

- images - APIC tag adds image support, yet most software barely reads cover APIC, and ignores remaining APIC (like label logos). Not to mention that this is lost space as each song in album will contain own APIC, making a duplicate of images.

- label tags - Some people like to listen music based on the label that published it. UKF Dubstep, Monstercat, or Ninja Tune are just some labels that people keep track of.

- alternative versions - some songs will have alternative versions, I don't mean remixes or covers by someone else, but main artist will often release few copies of the song. Sometimes the difference is in swear words (explicit vs censored), othertimes it is matter of length (original 7 minute mix vs radio 3 minute mix), other times it is matter of single having vocal vs instrumental versions. Sometimes it is a matter of live recording (ie: mtv unplugged). Most software will either assume songs are different because title tag is different, or will assume they are exactly same.

- sort tag - artists have one name they are to be displayed while should be sorted differently. For example: The Beatles should be under letter 'B'. Most software sucks at this, even though we have the tags, they are not there.

- album subtitle - some albums will have CD1 and CD2 with a subname. For example two cd album 'Destination Lounge: New York City', has CD1 subtitled 'Relax' and CD2 'Revive'. Most software either will need to drop the per cd subtitle, or will split and consider CD1 as separate album from CD2.
http://musicbrainz.org/release/cec1efa8-6541-4f9c-8f55-c86b1...

These are just some off top of my head.