My only concern really, since you addressed this:
I think they should be a new type of list field that uses a simple edit control instead of a list editor (like Keywords uses, etc.)
Is ease-of-use when tagging in the Panes.
I use the Panes to tag my files heavily. One thing I do a lot, is "fix" the [Artist] tag using the Panes. Tori Amos is actually a perfect example. All of my single-person Artists are in my database in the format
Last Name, First Name. When I import a new Tori track from Amazon or whatever, it is inevitably tagged "Tori Amos" and not the "reversed" way I prefer.
Having to (remember to) uncheck the previous tag could be troublesome, and would make any such operation twice as long. Mostly, it wouldn't be a big deal if you can see both checkboxes in the Panes at the same time (changing "The Arcade Fire" to "Arcade Fire"), but often that isn't the case for me unless the person's names both start with the same letter.
I'll constantly forget to do it, and then Tori tracks will end up in there both ways. Maybe that isn't bad, but then I don't want the Rename, Move, and Copy operation to name a folder "Tori Amos; Amos, Tori", and maybe a different one "Amos, Tori; Tori Amos" and so on and so forth.
So, I'm worried about the workflow of tagging. The VAST MAJORITY of my tracks only have one artist, and I only want Movies (not Music, not TV Shows, not Photos... Only Movies) to be allowed to have more than one [Genre]. I worry about adding workflow complexity 100% of the time to solve something that only affects my library 5% of the time.