I don't know if the 1.2 beta has entered freeze, but I have two suggestions for the UI, and more specifically, the track browser.
1. Allow an arbitary (in a reasonable limit) number of panes. For example, a two pane layout with Artist and Album filters. Or a four pane layout, in which case, the additional pane can select any kind of tag as a filter. Example of filters in a four pane layout: [Mood] [Genre] [Artist] [Album]. But for my personal usage scenario (and that of some users that only use an Artist/Album filtering scheme) even a simple setting to remove the first pane would be sufficient.
2. Allow double-click and right-click actions on the track browser panes. Possible way to do it:
Right now, with a single click on any of the panes, the panes to the right go to "All" and display all results in the track list underneath (although rarely that fails and the tracklist stays empty)
By double clicking an item in a pane, the selected 'Play Now' action with all the tracks in the tracklist should be performed. By right-clicking an item, some select actions could be performed (or possibly, all the actions possible by the right-click menu for a track)
3. A minor improvement, at the bottom toolbar of the tracklist (where the number of files is displayed) a total length estimate would be useful
That's it. Keep up the good work.