Several ways, but one of the more popular complaints is that the DH allows the pitcher to hide from the other team, and not be held responsible for his actions. In the AL, a pitcher can intentionally bean a player (or back him off the plate, or any number of things) with no fear of retribution. In the NL, that same pitcher has to come up to bat later, and can get a taste of his own medicine from the opposing pitcher. That changes things a bit.
