Some additional information about the always-on display mode has been revealed in the reviews of the device.

It's always-on Apple Maps.
The always-on display can be used with the Apple Maps, Phone, and Voice Memos apps. The directions, arrival time, and distance remaining are the only things shown on the map. The interface in the phone and voice memo apps are dim.
There is a demonstration of this function in the video below.
The always-on display works with the virtual Apple TV Remote feature in Control Center, which allows it to be used in dimmed fashion without the need for an unlocked phone. The Apple TV 4K and Apple TV HD models are supported by the remote feature.

The Apple TV remote is always in the control center.
The always-on display has an impact on battery life, but it's not likely to be excessive given that the display ramps down to a low power state when in always-on display mode. The feature can be turned off in the settings.
The Dynamic Island replacement for the notch, an upgraded rear camera system with a 48-megapixel primary lens, new Deep Purple and Space Black color options, and more are included in the new features of the iPhone 14 Pro and iPhone 14 Pro Max.
Ian Zelbo, thanks for your kind words.