Hope this isn't redundant...
From a pseudo high design level, the app window I see as 4 sections-top (caption & tabs bars & toolbar), right (now playing & curr track info & player controls), bottom (player controls option) and then the rest of the window (left/main panel).
I like the right-now playing-curr track panel showing whatever is related to what is currently playing and the Now Playing list. It's all relevant and related to one another. Something NEW...the option to display artist photos with or instead of artwork for the playing track in the right panel. (I'm not big on album covers but at times prefer a changing artist picture--for variety--like Playing Track tab.) I also like player controls in the right panel (still related to the rest of right panel) and to free up more real estate.
I like the option to still be able to display tabs and toolbar in caption bar in order to "better" use real estate but it certainly makes sense that the default should be separate lines especially for a cleaner look and for new users.
Now...the rest of the window (left/main panel). The left navigator is really all about what to control on the main panel which is the tab page. Library, filters, playlists, library explorer, column browser--they are all about controlling the main panel (i.e. tab page). So what if the left panel actually was located on the left side of the tab page and only those applicable pieces were shown? For example, if the Auto DJ panel was shown, do you need to see column browser or library explorer, etc.--unless you wanted to change the tab to show something completely different.
If navigation wasn't always going to be in a separate LEFT panel regardless and separate of the current tab, it could then be controlled and displayed/hidden per tab.
I use library/filters, playlists and column browser A LOT...library explorer, not so much. Perhaps I want certain pieces to be seen on certain tabs but not necessarily the same things on every tab. Maybe I could click something to see filters when I want for a tab but I want column browser to be constantly visible (for the tab).
Status bars...right panel would have its own status bar with option to toggle on/off. Same for each tab (i.e. main panel). So the window as a whole doesn't need a status bar (I think); Status bars would be tied/parented to either a panel or tab.