The two big things to know about Batman: Arkham Knight are that you can drive the Batmobile, and there's a new villain -- the titular Arkham Knight -- created by Rocksteady, Geoff Johns, and Jim Lee. If you need more to go on, our own Brett Makedonski just previewed the game.
He had several screenshots in his preview and here are a few more, all in high resolution. Arkham Knight is looking mighty fine, and I'm quite liking the design of the villain himself. I've heard theories that it's really just the Joker under the armor, which, please no.