I don't know if you were looking for little features or big features, but here's one I've wanted (and gotten a bunch of +1s on before) for a good, long while. It is a little thing, really (no comment on the PITA level because who knows):
Allow us to Control-Click in the Panes to multi-select (ala List Type Fields) on any string-type field, and then just write the results to the field semicolon-delimited.
[Artist] and [Genre] are special, as you know. This is good, because it makes it easy to re-tag an improper [Genre] and [Artist] as needed, while still allowing people to use Multiple Genres and Artists as wanted, without messing up Tagging for the more common case. But, it is annoying when you DO want to edit these fields to include multiple Genres or Artists, of course. This would be a simple and intuitive compromise. (In fact, I know I accidentally do it over and over expecting it to work, and I think in previous threads, others did too.)
You can even make the regular List-Type fields work the opposite way, clearing previous contents if you control-click in the panes, if you don't think that'd be confusing (but it might be).
