Keaton's age, looks, everything, was perfect for the Batman he was trying to play.
Keaton's Bruce Wayne was 10x better than Bale's. He truly played the "nut"- a crazy guy who is so mentally disturbed that most of the time, he was almost forced to have to PRETEND to be Bruce Wayne while deep down he's just itching to go back to his cave to obsess over files about the nemesis of Gotham.
It's a shame that we had the greatest portray of the Joker ever on film in TDK yet we had a Batman that looked "cool" rather than scary and a Bruce Wayne with more wooden than a table staring off into space half the time.
Not to diss Bale, he's a solid actor, but to this day I still feel that he's more a "stuntman" actor that's great to perform all the action scenes he needs to do in the Batsuit but as Bruce Wayne he's just a softcore version of Patrick Bateman.
He does not have the craziness of Keaton, the soft psychotic whisper that Keaton does when he's in the suit. I don't mind the grunting voices when he's up against enemies but when he's talking to Harvey or Gordon I think that's too excessive.
And who cares if Keaton wasn't as hulking as Bale? Back in 1989 less people were aiming to be as muscular as we are today. We were not as obsessed about our bodies and bulking ourselves to have huge biceps and so forth. The whole point of the Batman is that he is a physically fit guy who who relies on his detective skills to win his battles.
Keaton's Bruce Wayne was shown to stare tirelessly into monitors digging up old news clippings, fixing batmobiles.
While Bale's "detective" story relied on some lame GIMMICK having to do with a bullet that led him to a room (did I even get that part right?)? Sorry but Bale's characterization, "fingerprinting a bullet" did not add much graceful progression to the plot of TDK. I felt Nolan tried a little too hard to make a detective out of Bruce and included some bad CSI scenes that I would like to forget.