The 12 MP auto-focus camera is a honker. As we've seen in other smartphones, the thickness of the camera drives the thickness of the phone.
According to Dr. Hubert Nasse, the lens inside the camera module is composed of five individual aspherical shaped optical elements. This complex shape facilitates very high quality pictures out of a very small device.
Nokia chose to have the camera protrude outside of the back cover, which could either provide a good grasping point when taking the phone out of your pocket, or make it a hassle when returning the phone to your pocket.