Mortal Kombat vs. DC Universe fatality (seen at about 3:40 into the video after the break)? Well, nothing per se, it's a perfectly silly, violent way for the criminal mastermind to finish off his foes, and is actually surprisingly brutal for the game's Teen rating.
No, the only real problem is what the attack isn't. As fans of The Dark Knight know, the Joker's real fatality is blindingly obvious. Because, honestly, who doesn't love a good magic trick?