Too exasperated to even anatomize here, but let's just say that directly adding a new item to, say, the n-th place in a playlist on your ipod is now impossible. That's a deal-breaker for me.
More generally, at every turn, any attempt to get a clear picture of all of your media at once (even on your main computer, let alone on that and (in a separate window) any other device you may have) is frustrated. Even the ability to navigate between songs at the level of their information has been removed (so, e.g., if you want to enter the track number and name info for an album you have to exit from the informational summary for one file, go back to song listing then choose the informational summary for the next file, and so on: what was formerly a one click operation now takes a 4 or 5 steps. Entering the info for a whole album that used to take 10 clicks, now takes 40 or 50.)
In sum, a program that has up till now been unwieldy and irritating, has with version 11 finally become unusable and non-functional. Sell Apple now. The company has completely lost its mind.
Update: Having slept on it... things aren't quite as bad as I thought. For example, the multi-information I mentioned seems to have more contextual than I realized - you can't manoeuvre at that level in some playlist views and in artist view, but otherwise it seems OK (maybe the database has caught up with the problem overnight - it definitely seemed to be a more prevalent problem yesterday). So... grumble grumble OK. But another big problem: I've found that the search doesn't work as it should. I have songs by Annie Lennox both on her own albums and on compilations like Red, Hot and Blue, yet searching on 'Lennox' or 'Annie Lennox' no longer brings up the latter. Staggeringly, I can get the latter to come up when I home-share my collection and search using the older itunes on the other device. So in the strongest possible way iTunes 11 has gone backwards. That's unacceptable, dismal, etc.. Yes iTunes probably is improved in various ways but, like a car with all sorts of arguably interesting new features that nonetheless runs off the road or can't take a corner, it's still a piece of shit.
Update: To get iTunes search to behave reasonably, go to the search box and uncheck 'Search Entire library' and go with just 'Filter by All' checked. With that and the sidebar permanently turned on things feel almost back to functional (except for the no multiple windows problem which inflicts inefficiency across the board, presumably on the theory that inflicting what's optimal for ipads/iphones etc. on every other environment is acceptable now).