The first thing you notice about the game is that you do NOT start as Batman, but the professional thief Catwoman. Although it's just a simple mission, you realise from the word 'Go' that this game has a lot of excitement to offer. Thereafter, as you are still being introduced to the game, you play as Bruce Wayne, that too, in handcuffs, as you battle your way out of Arkham City prison.
One more noticeable change early on is a better gliding system. You have better control and far more options while gliding. Also, looking at the title, doesn't Arkham City seem better (or rather seem to have more opportunities) than Arkham Asylum. Well, in Arkham Asylum, you mostly are fighting inside buildings, with the occasional outdoor scene. Here in Arkham City, you have the whole city to explore, with thugs 'patrolling' the roads. Never try and fight them off, there is unlimited supply of thugs, so it's better to admire them from the rooftops.
A better combat experience also awaits you. With different unlockable moves available separately for Batman and Catwoman, you have quite a mission ahead of you to finish it all off in style. You can also play as Robin in the challenges, but that is available as separate downloadable content.
What makes this game even better is that you have a bunch of prominent villains awaiting to greet Batman: Joker, Two Face, Penguin, Quinn, Bane, Poison Ivy- you name it, with each one having an attached plot, with the two main battles of Batman vs Joker and Catwoman vs Two-Face.
I had finished each of Call of Duty: Modern Warfare 3 and Battlefield 3 in 5-6 hours, but about 3 hours of Batman has made me realise it a different ball game altogether. It is not so easy to conquer Arkham City, but everytime you decide it's enough for today, it leaves you yearning for more.
P.S. This is my first post after switching to the new blogger interface, and it does seem cool. Although a bit similar to the new Gmail, it provides everything you need at the exactly correct place.