* There is sort of a gap in the menus, where the gray area on the left doesn't reach all the way up. Not sure if that's intended or even fixable, but it struck me as odd.
* The scrollbars are nice, but the arrows' high contrast makes them stand out too much. Smaller arrows and a lighter shade maybe?
* The collapse/open indicators (i.e. > and v arrows) in the tree view look odd, too. The three main categories (Library, Playists, Computer) have nice, unobtrusive arrows, the items directly under Computer have a huge, bulky solid black arrow, and the folders under the drives use little plus/minus buttons.
* The active tab gets a dark gray, while inactive tabs get a light blue. With two tabs, one tab gets a signal-color, the other bold text... kinda confusing. (Also, there's something odd about the tab corners. Might be a general issue tho and not related to the skin?)
* The right part of the player trackbar uses an orangish color that appears nowhere else. Looks nice, but a bit out of place...
Still, great work. The composition with light gray backgrounds and dark gray accents creates a very tidy and intuitive interface and gives off a polished impression.