Since you are asking for feedback here goes...
The top in it's current form is annoying, but you've already said that will change so I'll leave it alone. One thought that I had that I haven't seen mentioned is why can't the volume slider be vertical instead of horizontal? I usually think of turning the volume up or down, not left or right.
The action window is a good step, but needs to evolve further. The main benefit is being able to see the songlist while your bopping around to other parts of the Application.
I know you may be sick of hearing it by now but the screen shot posted by knickelfarz and reused by others is very appealing. I would even take it a step further and move the tabs to the tab and have them control the whole right side of the screen.
The tabs would be on the left right beneath the player controls.
Beneath the tabs would be the tree or some version of it that could be fully collapsed to a single line if we didn't want to see it.
Below that would be the "action window" which likely be used for the playlist mostly because the Tabs at the top could take you to everything else.
The tabs in the mockup by knickelfarz could be expanded to include tagging mode, pane view, tv tuner, DVD playback, burner, visualizations, photo specific view, video specific view etc.
What you end up with is a very clean interface with a player at the top, a list of tabs below that on the left that control the view on the right and your current playlist below the tabs.
I know this is kind of wordy, and I'm sorry that I don't have the skill to create what I'm picturing, but it's the best that I can do at the moment.
I also am aware that this would involve much more work than adding the action window, but something like what's described above is where I'm hoping that Media Center is going....
As always, thanks for listening.