I installed the WMP10 Tech Beta on my workstation here at work.  At first I was mildy annoyed by some UI bugs, but it is a beta and I have faith they will hammer those out.  What really made my day though was a new play list feature.  I had added a carefully chosen subset of my music collection and was reading some site-o-links.  I clicked a link and it turned out to be a video.  The video promptly played in WMP, destroying my play list as has happened so many times in the past.  But, lo!  There was a little line of text saying I could click 'previous' to return to the play list that had just been destroyed.  This made me indescribably happy.