Clooney is a good actor and he mentioned his appreciation for Batman in his awards speech, but regardless of his demeanor, in no way, shape, or form is he a good Batman. He is far too soft. He fits the type of Bruce Wayne that Batman TAS created as a mask to the real Bruce, but I'm not sure Clooney has the gravitas to pull off Batman(regardless of the direction or script).
He has no rage to channel, or rather, I don't think his attempt would grant believability. He's too easy-going. Even in Syriana, I never really bought his performance as a CIA agent. Some of the things he did for the role were impressive, but he's never struck me as a "man's man". That's essential for the character of Wayne/Batman, which I deem the American interpretation of Bond in some aspects.
edit: he's at his best in the small off-beat flicks.