Being lazy, I never used to organise my music. Windows Media Player 9 changed that with the Media Library. It automatically found my new music, added album art, categorised it, and even brought up music reviews. Fantastic stuff.
But, in my attempt to remain cool, I've bought an iPod, meaning most of my new music is being encoded in AAC, which while supported through a codec for playback, isn't supported within the media library.
Which makes me very sad
So, single thing needed in WMP? Open it up to different formats natively. WMA is fine and dandy, but give us choice.