It turns out there is a way to show a sidebar in iTunes 12, however.
No, the new sidebar is not going to behave exactly like what users were accustomed to before in prior versions of iTunes, but it does work to be able to easily move media around between iTunes and iOS devices, and allows you to quickly jump between playlists, devices, and your stuff.
1. Open iTunes as usual to any media player screen
2. Click on the “Playlists” button (it looks like text but it’s actually a button, much like iOS)
3. The media view switches to ‘Playlist’ mode and a sidebar appears on the left, switching out of Playlist mode will hide the sidebar again
You’ll find all of your music and media playlists in the sidebar, but if you switch out of Playlists view, the sidebar will disappear from iTunes again. Thus, if you want to always see the sidebar in iTunes 12 you’ll need to stay in Playlists view, or at least switch to Playlists view when necessary.
In the default view, no sidebar is visible:
In the “Playlists” view, the sidebar becomes visible:
If the appearance of iTunes looks a bit starker here than what you have on your own Mac, it’s due to the increased contrast option being enabled for the broader interface of OS X.
This new sidebar functionality applies to all versions of iTunes 12 in any version of Mac OS X (or Windows). Since iTunes 12 is the default in OS X Yosemite, there is no option to use a prior version of iTunes, but for users on OS X Mavericks and prior versions of the Mac operating system, staying put on an earlier version of iTunes is possible – just note that eventually future versions of iOS will almost certainly be incompatible with the older iTunes releases.