Have you ever been playing one of your finely-tuned playlists, blissfully grooving to the music, only to connect your iPod or iPhone and lose your place? In other words, iTunes (if you've set it to automatically sync) will move your view from whatever playlist you're in to the sync window. That's handy, of course, but if you forgot which playlist you were in, it can be a pain. I have several playlists set to random, and when I try to go back, it re-shuffles the order. There are other multi-tasking issues with iTunes that largely stem from its one-window interface. If you want to browse the store and manage a playlist, for example, you're stuck doing a lot of scrolling. But iTunes isn't really one window!
If you double-click the title of a playlist, it allows you to edit the name of the playlist, right? But if you double-click the icon of a playlist (for a standard list it'll be a little blue note to the left of the title), iTunes will open an entirely new window, as seen above. You can open a bunch of these if you like and it works with TV, Movies, pretty much everything on the left side. It's much easier to drag and drop songs this way if you have a bunch of playlists. Plus, the window menu will give you quick access to whatever you opened, or you can switch through them with Exposé.
Note: yes, this has been in iTunes for a long time.