Mortal Kombat vs DC Universe, you see, has been scaled back in order to keep the dreaded 'M' rating off of the box. According to Kotaku, who spoke with Midway's Ed Boon, Mortal Kombat vs DC Universe has been submitted to the ESRB three times, making minor changes each time in order to move the game from a Mature rating to Teen. The most painful fatality in this process (pun intented), is that the Joker's awesome, awesome "bang" fatality (see it after the break) has been changed, as the final headshot is now performed offscreen.
So, in a nutshell, the series that spawned the ESRB is now bound to it. Midway needs its money though, and a T rating will sell more copies, which explains why the company is so serious about it.