NOTHING FOR ME. Here are the reasons:
1- Most important. I have 1862 books or documents on my iPad (epub and pdf). I regularly delete some and fill in with new. The new iTunes does not allow me to seek among the books in the iPad, it always seeks in the library of the Mac. To understand what I'm saying let's say I have a book called "New iTunes sucks" that I have on my hd and want to transfer to iPad (iBooks). Now I cannot check to see if the book is already on the iPad. This is a real game killer for me because I have over 23.000 books/documents on my hd that I may, at some point, want to transfer to the iPad.
2- As mesomorphicman mentions in his comment, the missing icon preview forces me to take extra steps in order to change the album/book cover, This is an issue because I have almost a terabyte of such files. Practically all the songs I transfer from my CD collection have no cover art.
3- After I start playback I always minimize the player. It takes very little space on my 13" screen.In iTunes 10 it was possible to change the volume from the mini player. The new mini player lost this and now shows a tiny pic of the album cover. Wow, if I was 16 I would probably be ecstatic about this rubbish.
I could go on but the above 3 issues kill iTunes 11 for me. Moreover, I'm afraid I will not be able to upgrade to the next OS (10.9) because it will install some future version of iTunes 11. So serious are the above mentioned issues.
Also I am very angry at Apple's attitude towards me as i customer. It should allow me to change back to the previous version if I'm not satisfied. As it is I had to look and find version 10 on the internet. It was not easy to find. Then I re-installed mountain Lion. What happened was that the OS was re-installed but it kept iTunes 11 !!!. There was no other way to get back to 10 but use time machine and restore the whole system as it was 2 months ago when I made the last backup. Writing this makes me extremely angry and frustrated. I BOUGHT OS10.8 so I PAYED for iTunes 10 which was included. Why am I not allowed to use it if I want to?
It is another thing if a software has bugs and another thing to submit to an almost fascist dictate from Apple on how to use your hardware/software.
I am a very disappointed customer ( exclusively Mac user since 1987).