This might be a bit esoteric but one thing I've always wanted is a "Date Rated" tag. Here's the situation:
I've got a ton of music in my library that I imported ages ago but haven't had time to listen to. Occasionally I'll use shuffle or Play Doctor to listen to those tracks that I haven't heard before. Sometimes I'll hear a song that I really like a lot so I'll rate it. Now, this isn't a problem if the track is relatively new and I've recently imported it because I use a "Newly Imported/Rated" smartlist that filters by Date Imported and Rating. However, if the track was imported a long time ago, after it gets rated it inevitably gets buried again and it's nearly impossible to find.
If I could make a "Newly Rated" smartlist and sort by the date that the song was rated I could easily have a list of all the songs that I've recently stumbled across and really enjoy.
Of course, sometimes a user might rerate a song and if he/she is "down-rating" it, they likely wouldn't want it in the Newly Rated smartlist. So it might be necessary to have a "Date First Rated" and a "Date Last Rated" tag for that purpose.
Thanks!