I'm sure you're sick of hearing it but I have to say it anyway - integration with MC 13 would be HUGE!
Failing that, it would at least be nice to be able to add subscription artists/albums/tracks to my library without having to download them. What I really want is a way to augment my existing collection with stuff I don't have. Ideally, I would be able to select an artist/album/track, right-click, and say Add to Library.
I also agree with previous comments that it would be nice to improve the browsability to discover new music. When I start up, I find myself scratching my head trying to think about what I should play. And there is no easy way that I can find to get a variety of music started quickly. The Artist Radio feature is nice, but the artists are too similar. I would prefer to be able to specify any number of artists, or any number of genres, and start playing music based on those selections.
This might be getting crazy, but taking the above one step further, it would really be cool if it would scrobble my library like MC with the last.fm plug-in and then play similar artists, and tracks I don't have from the artists that are in my library.
I have also always wanted to be able to create Smartlists based on percentages. So I could set it up to play 50% Rock, 25% Pop, 25% Alternative for instance.
In general, I think it's a great product. Thanks for listening.