What I like:
- the "Up Next"-Feature is quite fancy, I like the thought on making a quick playlist that gets deleted once it's finished
-performance is better: the app loads more quickly, the database is available faster and it hasn't crashed on me yet
-the bug where iTunes crashed because of a "problem with the iTunes Audio Scrobbler" has apparently disappeared
What I don't like:
-there's too much iOS in there. I'm a classic mouse/keyboard user and the UI at times feels like it was made for use with the iPad/iPod touch/iPhone in mind, which I can somewhat cope with, yet I feel it should have been geared more towards non-touch interfaces
-I miss the bar on the left side. Doing drag & drop when making playlists was something I really cherished about iTunes 9 and 10
All in all, I think iTunes 11 is pretty neat, but the UI was too much of a break with the formerly employed UI concepts.