I actually had this problem with The Dark Knight. Diligent use of profanity and blood in a few key scenes - just a little, nothing gratuitous - would have heightened the realism for me, as well as making the movie feel more like the adult, Heat-inspired thriller it was aiming to be.
1) The chase scene. As the helicopter crashes, the cop says something along the lines of "oh, that's not good", when in reality people curse in traumatic situations. It didn't feel true to life.
2) When Batman is bashing the Joker around in the interrogation room. The bloodlessness of this scene always seemed odd, but a bit of blood would visually highlight Batman's violent methods and would make the audience feel like he was perhaps taking it too far.
Ironically, I think one of the effects which nearly pushed the film into R rated territory, Dent's disfigured face, was too graphic and looked unrealistic as a result. Look at images of any recent burns victim - The effect should have been far more subtle.