In AO you get to visit Arkham City before it became Arkham City. Basically, it's the same map (a bit extended), except everything looks normal. Seeing some locations in their "glory days" is fun, but not enough to overcome the feeling of deja vu. I had expected to see people (of the non-criminal variety) in the streets, since it was supposed to be a normal part of town, and on Christmas eve, no less. That would have introduced true novelty into the game, but instead WB use the cheap excuse of a city lockdown to keep civilians off the streets. There seem to be fewer surfaces you can grapple onto, so moving around is not as seamless as it was in AC. Also, there are respawning snipers that are a real pain in the butt and further restrict your movement.
Of course, there is a brand new story, and it's pretty good, even great at times (bonus points for referencing The Killing Joke). A lot of effort is put into character development: you get to see a younger, more radical and dark Batman, and his transformation into a more mature man, as well as the beginning of his complex relationship with the Joker. On the whole, however, the story feels somewhat anticlimactic, and the confrontations with some of the major villains are not even part of the main quest.
The combat system has been revised so that now you cannot counterattack while in the middle of attacking someone. It may seem like a slight change, but it makes the combat a lot less fluid. On the other hand, you become so overpowered toward the end of the game that nothing short of a small army can be considered a serious threat. Here, once again, WB are afraid to tread new ground and give the player what they think he wants, even though it makes no sense for Batman to be more powerful in the beginning of his career than at its peak. A bolder developer would have given Batman fewer gadgets and placed more emphasis on stealth, but a bolder developer WB ain't.
The multiplayer also deserves mention. It's very similar to Spies vs. Mercs in Splinter Cell: Blacklist, only it's more like Spies vs. Mercs vs. Mercs, with Batman and Robin being the spies, and Joker and Bane's men being the mercs. As Batman/Robin, you can zoom around the map and take refuge on high points, but are very vulnerable to gunfire and have to be stealthy to survive. As one of Joker's/Bane's men, it plays like a 3rd person cover shooter, where you have to watch out both for Batman/Robin and the opposing gang. You can also get the opportunity to play as Bane or the Joker, who pack an extra punch. It's fun, but there are only four maps, and Spies vs. Mercs feels a bit more thought out, so I would recommend Splinter Cell if you are into that sort of thing.
The review wouldn't be complete if I didn't mention the bugs. It's normal for a game this big and complex to have some, but when they render the first major fight (which happens very early in the game) close to unplayable, that raises some questions. And, of course, there is the infamous Burnely comms tower bug that makes it impossible to unlock a portion of the map for fast travel and very difficult to complete the Enigma side quest. I was thinking of giving the game a 7, but after some consideration, it doesn't deserve more than a 6, not with this kind of issues.