For a new interface, I realise it'd be hard to please everyone so how about doing a RAD approach.
Make 2 or 3 interfaces, they dont have to work as far as fully managing the music goes, just have to read the db and let us browse it so we can test it. Make it in something like VB or Delphi so that you can really throw them out fast.
Then put those on the site and let everyone download them and play with them and see which interface they like the best.
Then get feedback on the things people do and dont like about each one, combine them, throw out another quick design to test it and so on.
Hands on using is the best way to decide what's thought of an interface. Talking with no physical item to talk about just gets confusing and often means the idea isn't explained clearly.
This would let you really easily design a new interface without worrying about messing up the functionality of MJ.
I also think that things like: Analyse Replay Gain, Tidy File Names, The plugin's like find duplicates etc should all be put into a menu item called: Music Tools or something similar. The plugins should be able to install themselves into this.
Right now they can be found through properties, but if someone was just using MJ for the first time and wanted to do these things to all the music in their library, they wouldn't think of them as properties to a song, more as tools for their library and thats where they'd try to look.
Just a couple of ideas
Good luck