I tend to ignore the constant updates Apple does to iTunes. The reason is the reputation of major problems following updates. This could be solved if there was a different system in place. This is the only software I know that makes you install the entire 200 MB package all over again.
So here's what happened to me. After several months of ignoring the update, I finally installed it a couple of weeks ago. Then yesterday I bought the iTunes LP "The Resistance" by Muse. (Amazingly the extra stuff like photos, videos & lyrics took longer to download than the entire album) Fine. Then I burned a CD like I always do. The reason is to make sure I have a backup. Plus it's an audio CD I can play, so I know the download actually worked, & I can re-install it if something happens to the computer.
Again this worked fine. Then I plugged my 4th generation nano into my computer. Not so fine. It never showed up in the sidebar. So I unplugged the nano, plugged the USB cable into a different port. Still nothing. Went online, looked for info. Lots of convoluted explanations of what to do. Finally shut down iTunes & restarted it. Then it worked. Whoopee!
All I can guess is that I haven't syncronized my iPod in quite a while, & certainly not with this version. This also is not the first time this has happened. It seems to be a problem with new downloads. If I add a Cd in my collection it doesn't seem to do this. Meaning that strangely, iTunes sometimes has issues with it's own files, which doesn't seem to make sense. It should have more problems with outside devices like CD's,especially since you don't have to be logged in to upload a CD.
I still contend, although Apple would disagree, that the music store should be separate from the manager/player. Put the store online, meaning make it a website like Amazon. After all, you can download from Amazon & have the file sent directly to iTunes, so that shouldn't be an issue.
Once you do that, iTunes becomes a smaller program, with probably less need of absurdly large update files. The use of Quicktime appears to be an issue to some people. Not sure if thats my issue or not. Not popups saying it is, but I notice in the support forums that it is for a lot of people. Remove it, either use the hated Flash (I've never had an issue- all video works fine on my computer), or HTML5. Size diminishes, meaning less problems.