Robin plays similar to Batman and Catwoman in the basic mechanics. You can attack and counter-attack like you normally would, but Robin has access to some different gadgets than Batman. He attacks with a bo-staff that can expand into a shield, allowing him to move slowly towards enemies with guns. He also has a snap flash that can stun as a timed bomb. His other gadgets are all variants of the normal arsenal Batman has, including a stylized shuriken that acts like a batarang, and a grapple hook that launches Robin towards the enemy for a zip-kick, rather than pulling the enemy towards him like Batman.
You’ll get more of the high-quality voice acting and detailed visuals from the narrative of the main game. There’s not as much mystery in the story as there was in Arkham City, and the ending seemed a bit anti-climactic to me. It would be nice if the writing was a bit more complex, but more of the fun gameplay of Arkham City is great. I actually found Harley Quinn’s Revenge ran better in Direct X 11 than most of Batman: Arkham City. If you played the main game in Direct X 9 for the higher framerate, I’d recommend trying Direct X 11 for the DLC, I barely ever dropped below 55 fps, even in fights where there was lots of action going on.