This game is indeed something special; it was the first sandbox game I never wanted to stop playing. I've played good sandbox games before: Infamous, Red Dead Redemption, but this was the first game where I didn't get bored and felt like stopping by just traversing the game world. Arkham City has blended a fantastic movement system with seamless combat to create an experience that is breathtaking. The story keeps the a powerful focus on the cast (it does get a little crazy and weak with a few plot twists that occur later in the game, but the ones that count are executed fantastically) while serving as a great commentary for the relationship between Batman and the Joker and how much the two are interdependent on each other. It sounds weird, but it won't once you experience the story.
The boss fights are vastly improved as well, with a big shout out given to the fight with Mr. Freeze that perfectly reenacts how Batman takes on villains who rival his intelligence and outperform him technologically or physically. The predator sections in the game didn't feel as strong as the first one, which is concerning since they are more complex this time around, but then again I might have been less creative this time around. The adjustments to the combat take a moment to get used to you if you're an Asylum veteran, but are great and intuitive and vastly improve an already perfect system. The challenge rooms are great and the Riddler side missions (riddles, trophies, puzzles, etc.) never got boring: I completed all 400 in 2 days, because it was that addicting.
This is a sandbox done right. This is a video game right. This is Batman done right.