+1 for the idea to put the columns in artwork mode too.
Some more requests:
If the number of tracks is displayed in the Track Browser, there should be the option to have those numbers right-aligned.
This way they won't interfere too much with the rest of the text.
Maybe there could also be the option to have the number of albums or artists displayed instead of the number of tracks.
I think it could be pretty comfortable to have the Main Panel remember different layouts for different parts of the library (e.g. music (including different filters), audiobooks, inbox) and for different tabs too.
For example: Right now in Music i use the three columns genre-category, genre, album-artist whereas in Audiobooks i only need album-artist. It's kind of annoying to always have to manually switch the layout of the Main Panel.