Windows Media Player 11 had a very clean and streamlined look. One reason for that was the non-inclusion of column dividers and the autohiding of the one for the Navigation Pane. Windows Media Player 12 cluttered things up by changing this.
Get rid of the dividers completely to make things clean like they were in 11.
Alternatively - Autohide them and only show them on hover, the way the Navigation Pane is currently set.
Dividers highlighted in red in picture, blue in actuality.