I think the best way I can answer this is simply by advising you to read Batman: Black & White. Most of the stories in there will do. The one that comes to mind, my personal favourite, is one that simply involves Batman performing an autopsy on the corpse of a woman. As he does this, he reconstructs her rather unpleasant death. The story is strong for Batman's monologue, but part of what makes it work is it's frank depictions of violence and the reality of what Batman is dealing with: a murdered woman.
That reality won't be seen in a PG-13 film. The true face of crime in Gotham won't be seen, nor the true toll on the victims. There is a weight that will never be there, by virtue of the violence and, more importantly, the subject matter being glossed over.
This doesn't mean you need to show Joker splitting people open. You don't, and mindless gore isn't the point. The point is presenting material that you couldn't present otherwise, and presenting it in a straightforward way that hammers home the appearance of real crime and real suffering that communicates to us, the audience, what the reality of Gotham City would be. Through this, you more precisely and more convincingly define exactly what Batman is fighting: not thugs, not supervillains, but suffering in it's most concentrated form, in a city where it is more potent than anywhere else on Earth. The adversary is brought in to sharper focus, and we better understand the sheer magnitude of Batman's mission, and of what he faces.
This is not necessary or even desirable for all Batman stories. For others, though, it is, absolutely. When you can't address this material, can't even talk about it, you shelve these tools. Depending on the story, it may not matter. But they are useful tools, and they should be accessible if a filmmaker wants to put them to use. Unfortunately, this is not the case, as per Warner Bros. PG-13 mandate. If we learned, somehow, that WB had no such mandate--that Nolan would be allowed to make an R film--and the final film still ended up being PG-13, that would be fine. That would mean Nolan had access to every tool, and picked only the ones he needed for his story, not needing the R tools.
As it stands, though, Nolan only has access to two-thirds of his tools. As a result, we have no idea if there are ones he'd like to use, but isn't allowed to.
t:
- Ahahahaha!
