Naturally, not everything is perfect with iTunes 11. Many people seem to have lost album art; in my case, all of the art for my movies and TV shows disappeared, and dozens of my albums lacked art. (See this hint for a way to get iTunes to redisplay art for movies and TV shows.) In views other than Albums, you cant click an album to select it and then (for example) press Command-I to change its tags; instead, you have to select the tracks -- and if they're in several columns, doing so is harder than it used to be. Some animations, such as when I created a new playlist, lag more than they should, even on a Retina MacBook Pro. And surprisingly, iTunes 11 doesn't hook into Notification Center. People will quickly adapt to many of the other small annoyances, but in other instances that they'll continue to miss the old approach.
A number of things have been cut from iTunes 11. For example, while you can go to List View for your music by clicking Songs, but you cant display album art in that view. Album List view, which was in iTunes 10, is gone. The iTunes window no longer includes an album art window at the bottom left. Instead, the album art for the currently playing song displays in the iTunes LCD (the top part of the window, with the progress bar); and if you click the album art, a larger window pops up. Also, you can't have multiple windows in iTunes. In the past, many people used a separate window for the iTunes Store or for certain playlists, but that's gone in iTunes 11.
Other notable MIAs include the Advanced menu; the Find Duplicates command; the Gapless Playback tag (which hardly anyone understood anyway); Cover Flow view; the Column Browser (other than in Songs view); and the iTunes Sidebar, which displayed iTunes Store and Genius information.
All in all, version 11 is a good upgrade to iTunes. Some parts of the program have been simplified, while others see new complexity. With the focus on playing instead of organizing, Apple is trying to turn iTunes into a more user-friendly tool for playing media.