Most recommended tag fields to use?

First off, I love MP3tag, and I've been using it for about 2 years. I'm casually but fairly knowledgeable on how it works.

I'm on staff at (OverClocked ReMix), where video game music fans arrange the music of games in interpretive styles and we release it for free to educate people about VGM.

I'm in the process of retagging our 2,000+ archive of MP3s to make everything more consistent and compatible with most major music programs and players, and really wanted to hear from some experts as to what data fields I may be overlooking.

Fields I use:

Album Cover/Art (embedded)

I've seen other potentially useful fields, but I've literally never seen them used from major of MP3 program (in my case, Winamp & iTunes) or MP3 players (in my case, an iPod).

Are there other fields that you would recommend filling in, and what applications or players regularly read from those fields? I'm wanting to make sure we don't have blank data in major data fields or any major MP3 players or programs, like through foobar, Windows Media Player, Zunes, or anything else that is used to some decent degree.

Fields I didn't plan on using:

BPM: Would love to add this for everything, but don't know of any prog that is 99% accurate determining BPMs. The only good, free prog I know of is BPM Analyzer, which automatically adds the BPM tag to any MP3 you analyze. That said, it can still be wrong and thrown off by tempo changes, and I'd rather have that kind of information be very consistently accurate rather than spotty.

Audio File URL (%WWWAudioFile%): Looks great in terms of potential, I've just never seen an instance where that tag is read by an application.

Release Date: We often upload tracks in advance, so those dates sometimes might not be accurate. Thus, I've chosen to leave that blank.

If you want best compatibility, you should stick to the V1-tags. I know some mp3 players that only understand these and refuse to play files with V2-tags.

If you want best value for collectors, you should add all the information that you would put on a booklet of a CD or write down for a track, incl. lyrics, composer, participating artists and stuff.

As there are not enough fields for all the information (e.g. where do you put the recording date and place?) it may be worthwhile to think of a metasyntax how to separate one information from the other in more general fields like subtitle, unsyncedlyrics or comment.

Also, some players rely on special fields like "podcast" to identify a particular kind of audio.
So, IMHO, it is much more a question of the target group than a list of tag fields.