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As obvious as it may have seemed now, it certainly eluded the people behind the earlier films for at least a film or two longer than it should have. But Nolan garners "so much credit" for the melding of different graphic novels, exquisite casting, and overall execution, I think, and not just for deciding to go grittier.
I think it's generally an oversimplification when people refer to Nolan's Batman films as "grittier." They are, but they're not overly so. What makes them brilliant, to me, is that they still "feel" like other superhero films in that they've got the gaudy gadget and are widely accessible, but there's more there for anyone who wants it.
Anyway, I didn't find the shift in tone audacious; what I found audacious was the choice of villain, and the extended prologue to Wayne's becoming Batman. I think more than a few moviegoers were surprised at how un-Batman like the film seemed for the first 35 minutes or so.