What's most important to me:
- Smart Playlists: and an "And, Or, and But" option between each condition. For example, if I want a playlist that's artist is either Usher or The Postal Service (a horrible combination, but the first two that came to mind, lol), but are not from the Confessions CD, I currently can't - I can only match all (leaving no songs - I can't have two artists), or match any, throwing in Usher, The Postal Service and all my songs that aren't from Confessions. Grrr...
- Import album art for all the songs I have that have available counterparts on the Music Store
- Expand iTunes' Music Video reach - with Launch out of the question for Mac users, offering a similarly huge database that is searchable, and features a Top 10 TRL-style countdown would draw in traffic like crazy. Putting a video link beside all songs on the Music Store that had their video available would make life easier too. And the ability to stream the music videos from the store to the album art window in iTunes (with a fullscreen option and full-visualizer option) when listening to a song with a Music Store counterpart would be the kind of next step I expect. Of course they'd have to be fast - no delay or hiccups.
- As others have said, listing multiple albums for a song
- Playlist folders (I'd say it's pretty much guaranteed)
- The ability to only share checked songs
- The ability to clean up all the crap in my iTunes folder that I deleted but hastily entered "Cancel" when asked if I wanted to remove it from my library folder as well
- An official iTunes "TV Watch" section that put a weekly playlist up of the songs played on that week's OC, Alias, etc - I know iMixes are good, but this would be even better
- The ability to make comments to an iMix
I've had other suggestions over the last year, but they slip my mind now...