I think iTunes on the Mac is pretty decent (compared to the Windows version, at any rate. God, have you tried doing any heavy I/O stuff in iTunes for Windows?).
I'd love the ability to have nested queries in Smart Playlists. The only "solution" at the moment involves having folders full of Smart Playlists for individual queries and then having another Smart Playlist to assemble that "level" of the query.
I'd also like the Genius Recommendation feature to be a bit better. The Genius Playlist feature is pretty solid at the moment (when it came out in iTunes 8, I was pretty much blown away), but the recommendation stuff is still nowhere near as accurate as what it's actually trying to emulate -- asking an expert, "hey, if I like this music, what else would I like?"
Edit: Oh, and more album art in Apple's databases. The Gracenote stuff isn't exactly fantastic. I know there's TuneUp and stuff like that, but Apple boasts that it's a complete feature built in to iTunes, when really, it's not that comprehensive in my experience. Who knows, maybe I just listen to obscure music.