Well I haven't upgraded and am not using iTunes 11, however my sister is on her W8 laptop and I set it up for her, and just found it clumsy and confusing coming from iTunes 10.
Again I can't really say what is missing because I haven't used it for more than a couple hours to setup my sister's iPod, though I heard from others that some key features were removed, and I do know that the interface has been redesigned, and albeit very nice it is just too simple for me.
The one thing I did notice right away was that cover flow was gone, and I also dislike the new mini player. I hope Apple isn't getting ready to abandon the traffic lights!
It seems incredibly slow compared to v10, especially when adding info and really unintuitive. I was trying to set it up for my in laws and it was a bit of a pig.
I'm not sure who thought this version was a good idea but maybe they should join forstall on gardening leave.