I'll add a few things:
1. Overall UI design. Dialogs, icon choice, etc. are old and don't give the app the "eye-candy" appeal that it needs to attract windows users away from the free program MS gives 'em. You use the wrong metaphor icons while MC9 is working, you use a Search icon on your dialogs when it's processing updates to the database and tags.
2. Stability/Performance: Performance has to be improved. I know we're still in betas and all that, but the speed of startup and navigating tree items in WMP Series 9 is much faster.
3. Refreshing Tree's on changes: this has been the bane of MC9 as far as I'm concerned. Steve's iPod plugin does this , but the rest of the app doesn't. For example: if I have an Artist>Album view selected and I edit all the songs in the view to change the album, the tree should refresh and display the new album name upon Applying those changes. Right now the user must first collapse the tree item, then expand it again.
4. Don't hide features in menu options.
5. Smartlists are the best in MC9, but the interface on building them is behind the times. Look at WMP Series 9 smartlists. Their wizard is what you need to benchmark against as far as form. Stick with the function you guys have though--far superior.
6. Enable "Watch Folders". I think every media player has this option now.