ARS Technica had a quick look and noticed this upcoming media player features some surprising differences:
The UI itself is brighter and lighter than WMP11. Some buttons and toolbar items have been moved around, but the experience should be pretty familiar to users of version 11. What does represent a big change is the removal of the Now Playing button, which in WMP11 switches to a view showing the current playlist.
This is because WMP12 completely separates library management from what's currently playing, with two distinct player modes; Now Playing view, and Library view. Library view contains all the library manipulation features that should be familiar from WMP11. Now Playing view contains the current playlist, visualizations, and videos.