I still can't sort my music by artist's last name in iTunes, which seems like the most basic and most desirable feature you could put into a program like this. It is how almost everyone sorted their music before computers came along. Why is this feature not available?!. I'm sure the reason is that the underlying CDDB database doesn't have separate fields for Last name and First name, instead having only Artist. But come on, for something this important, with many hundreds of millions of dollars of market pressure behind it, this is a very easily solvable problem. Get on the phone, call up Craig Palmer over at Gracenote, and get it done.
There are no user-definable database fields. For example, imagine if you could use Genre for your own purposes, to put something like "mellow" or "road trip" or "dance" or "memories" other classifications that mean something to you, but aren't necessarily generalizable. This would be about 3 hours work for some programmer to add some fields to the database, and let you use them for searching, organizing smart playlists, etc.
There should be a "similar" axis that can be used to relate songs to each other. Imagine a play similar menu.
Play all versions of this song. I happen to have almost every recorded version of "Georgia on my Mind", and short of making a playlist, I can't play all of them one after the other.
Play songs I like but haven't heard for a while. iTunes knows the most-listened to songs, and ratings; seems like this should be easy enough.
All in all, I think iTunes is a big disappointment for organizing and listening to music.