I regret upgrading, and hope Apple walks some of these changes back.
* I can no longer open multiple playlists at once, or open the store and a playlist at the same time. I used this a lot. It was useful when organizing playlists, and essential when looking at what tracks were added to iTunes album re-issues so I could purchase just the new tracks, not the ones I already got from my CD.
* Search is painfully slow. On my 2.8GHz iMac, it beachballs for up to 15 seconds when I type the first character, and freezes for 2-3 seconds for each additional character I type. Search was instant in iTunes 10.
* When I switch playlists, it forgets where I had scrolled to and what genre, artist or album was selected in the Column Browser. Whenever I switch lists (which I have to do a lot because of the first problem) I start at the head of a long list.
* The album view is unworkable with a large library, especially with a ton of singles purchases. I bought about a dozen iTunes Essentials playlists, each of which adds 75 tracks from 75 different CDs. That's about a thousand album covers, each of which represents only one song. Yet iTunes keeps reverting to the album view no matter how often I pick the list view.
* Last night it downloaded several TV shows I subscribe to, but all the episodes were marked as "watched" while a bunch of shows from years ago show up as "unwatched."
* I gave up and turned off iTunes Match because every time I relaunched iTunes 11, it ran a Match scan that lasted about an hour and a half. iTunes 10 got through that scan in under 5 minutes.
* I miss the album cover in the bottom of the sidebar. The new one is too tiny to be useful, and doesn't change when I select tracks for editing while listening to other songs, or function as a dorp target. As best I can tell, I have to use the command-I dialog in order to check and edit album art now.
* Aesthetically, I don't much like it at all. It feels like Bing, the new Skype, or a Linux desktop app. It's very flat with soft shadows and gradiations as the only decorations. Controls appear on hovering over some items for a moment. There's lots of space wasted on gutters. I wish I could turn off that fat header on playlist views when I turn the status bar on, for example.