Command quit will close the iTunes window but won't quit the iTunes app
iTunes 12.1.0.50
Command quit will close the iTunes library window... but won't quit the iTunes app properly.
The iTunes icon in the dock still says the app is live... click the icon in the dock and the library window pops up again.
On the second try the app will always close properly.
iMac (27-inch Mid 2011), OS X Mavericks (10.9.1)