This is discussed here in detail (the entire series is highly recommended)
So I’m playing the rogue, and we all have quests to kill the leader of the ogres, or the ogre shaman (you had to murder so many ogres), who is in the back of the cave. So if you play a Druid or a paladin or a priest you have to kill his whole tribe just to get to him, possibly even wait for more of them to show up to murder them as well.
Meanwhile the rogue, who’s supposed to be the most ethically challenged class in the game, can sneak past all of his friends, kill the one troublemaker, and sneak back out without having to take any innocent lives. What kind of weird messages are we sending here, Blizzard?