Over 2.5 million copies of Batman: Arkham Asylum have been shipped. The game's really good. It might have been different.
According to GameInformer, the combat went through several variations: "The first one being a full rhythm action game! The second one was prototyped in 2D, which popped up whenever you got into a fight, and involved colored circles bashing into each other. This actually formed the basis of the final system."
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Rhythm...action...game? THANK YOU FOR NOT USING THAT COMBAT SYSTEM. More factoids and tidbits in the link below.
Tidbits: What You Didn't Know About Batman: Arkham Asylum [GameInformer via Joystiq]