I collect live recordings, so there's a bunch of less common stuff I use a good bit: Date performed, handle of who recorded it, tour, a personal rating system for the recording quality, additional artists (special guests + the like), and general comments I might want attached to the file. ("clips at 0:40" or "heckler in the quiet part"). I may have 20 copies of the same song, and I may even have multiple recordings of the same song on the same night from different people.

At least for some artists, there's a huge number of show recordings out there. The guy who maintains the (unofficial, although known and tolerated by the artist) live archive for Nine Inch Nails is up to >1,000 recordings, for example.

Now that I think about it it may have been that there was info in foobar's media library database that wasn't supported, not necessarily id3 tags. Play count and last played date are some I'm partial to that I couldn't get working.

I also use album artist a lot, though that's probably supported.