(I have yet to try out 4.0)
I fantasize of this taking over my media player, for all of my 4 computers. In addition to the want of Sorting by Artist, Genre, album, etc. I am proposing "pop-out"able windows. If you are familiar with winamp, it was multiple windows for each of it's tasks. It would be nice to see an option to "Pop-Out" a window, such as the current playlist and playback controls, or the music library. I don't know if this would cause difficulty with javascript, being able to call upon windows (not open, but send data from one window the the next, maybe cookies and a push system), but it would be a wonderful feature.
The reason that I thought of this, is do to my netbook, were I had to edit a theme to decrease the size of the font, be able to see what I was doing exactly. But if I were to be able have a separate window, it would be perfect.
What do you guys think? I know it would be quite easy with a programing language, such as C++ or VB, but I do not have enough experience with specific web-apps to know if it is even possible to send data between windows.