I use a bunch of custom fields, which are "almost" covered by the proposed new standard fields. But with some subtle but useful differences...
One underlying hope is that all fields, or at least all fields that don't break anything, could be more configurable, as suggested elsewhere by various people including me. Specify how a list pops up, whether stars or values are used, whether the field is read-only, derived via an expression, etc. Many of the desired capabilities exist in MC; the hope is to make them available for every field.
My custom fields:
Rank (instead of Rating) = I don't use the standard Rating field because I discovered it is too easy to mess up the value. One inadvertent click, maybe an unnoticed click, on the stars in the tag window or a column, and the Rating value is changed without confirmation. I spent a zillion hours rating my 80,000+ tracks and can't risk unwanted changes, so I added a custom Rank field that also accepts values 1 - 5. As a "normal" data field, changing the value requires a bit more action which protects it. My big hope is that the Rating field becomes more controllable/protectable so I can copy my Rank values back to Rating and use it. A big improvement would be to allow the Rating field to be set to display just the value rather than stars, so editing it behaves like other fields. And maybe provide this per-column so stars can be displayed read-only. If the Rating field can't be "improved", my second hope is that I could display read-only stars for my custom Rank field.
Artists (in conjunction with Artist) = One of my two MC power-tool fields. This is a multi-value field that lists all the notable artists on track. I use the standard field Artist for the exact artist name as stated on the recording, and Artists to identify the performers in a standard way across all their recordings, bands, etc. The names are in format "Last, First" so this becomes the consistent way to locate recordings by an artist who might have been in a variety of bands, or varied the band's name, etc. Very useful in Jazz and Classical, but also helpful in quite a few rock/pop/folk recordings. The standard Artist field, because it contains the exact artist as stated on the original recording, is nice to display during playback, but otherwise it's never used to group or select music. The custom field Artists does it all.
RecSource = CD, LP, 45, CS (cassette), AT (audio tape), VT (video tape), AC (aircheck), etc.
ID = Used for essentially the same purpose as the new UPC and Catalog # fields. My main purpose is to identify where in my shelves of stuff I got the tracks, in case I have to go back and re-rip. So I enter whatever info I have, preferring UPC but there's no such thing for pre-CD-era recordings, and I often don't know it for purchased downloads. So I use the vinyl album/single's catalog number, for instance.
RipInfo = Helps me know when I ripped, since it is often different than any date tracked by MC. Usually in format 20081016-MH (YYYYMMDD-initials), the same format I've always used for comments in source code of apps I've written.
Recording = S (Stereo), M (Mono), R (Rechanneled), 4 (4-channel), etc. This helps me realize when I don't have a desired recording type, and helps differentiate when I have multiple desired recordings of the same track, sometimes needed because lots of 1950s-1980s recordings were issued in mono and stereo but are different mixes, edits and/or takes (notorious situation with Motown, Beatles, Beach Boys, and many more).
RecVer = Something to distinguish multiple recordings of the same song by the same artist. The RecVer value varies depending, might be ALT or ALT1, ALT2, or LIVE, or whatever.
Performance = Vocal, Instrumental
Tempo = Sort of like fields Mood and Situation and others, just a basic way of knowing the overall feel of the track: B (ballad), M (medium), U (up-tempo)
Chart = highest Billboard chart position (using Joel Whitburn's book).
Keywords = My other MC power tool field. I use this standard field for a variety of dissimilar data that arguably could/should be better in separate fields, but since I then extract it via expressions and smartlists, tossing it all together works. For instance...
Decades -- I have a set of Keywords values for every decade (1990-2000, 1980-1989, 1970-1979, all the way back to 1890-1899). I also use the Year field for the year of the original recording, but I don't always know the exact year, so decades is a useful (if arbitrary) way to group similar music vintages.
Type of music -- Rather loose notion, but with many more values so much more precise than Genre, given the many sub-genres that might be used. Also helpful for other types of "find it later" keywords, such as "car" for songs about cars/driving, surf, live, summer, dining, motown, beatles (beatles songs no matter who recorded them), tropical (my wife's fav), italian, french, and many more. I use these values to build any desired type of smartlist.
Instrumentation -- I'm a musician so like to identify tracks that are primarily piano, guitar, sax, horns, symphonic, kazoo, whatever.
Purpose/Nature of music -- Another loose notion, but useful to identify christmas, comedy, children, novelty, historic, and all kinds of other terms that would help me locate certain recordings among tens of thousands.