I presume you don't use iTunes that much or you would have spotted them.
It starts when I start up iTunes and go into the iTunes Store. Every time it now makes my computer freeze for anything up to 5 minutes while the iTunes Stores loads. That never happened with previous versions. Then when you try to browse through the Store it's soooooo slow and laggy it's like wading through treacle. Pages take an age to load. Then you see something on the front page that you like and select it to get more details (say a TV Show). In the past when you load that page it would show that TV Series in the main window and on the left hand side it would show you all the other series available for that TV Show. That's gone now. So you click the name of the show at the top thinking it will load a page showing all the series but it doesn't it just shows the same page again so you have to go to the search bar in the top right and re-type the name of the show. Even then half the time it doesn't show you all the series available.
Searching is a nightmare. If you get the spelling even slightly wrong it doesn't find anything. That's just plain crap. When you eventually do a serach it comes back with lots of responses so you have to wade through each page. If you select something to see more details and then try clicking the back button it takes you all the way back to the start of the lists and you have to wade through all the freekin pages again. That is just bad design, bad coding, bad whatever.
I could go on and on about the other freekin problems with this pile of crap but I'm busy so I won't but anyone who uses iTunes 11 extensively will be able to tell you all the problems with it. And I haven't even mentioned all the useful features they removed with version 11 like coverflow, alerts, resizing artwork, etc, etc.