In the Outlands so far, there's been a persistent slavery sub-theme. The demons and the naga often have Broken slaves and they will force the slaves to attack you when you fight them.
Maybe it's just because I play a paladin, but I've always felt a little uncomfortable killing the slaves in places like Blackrock Mountain. I tend to go out of my way to avoid aggro'ing them.
So when I got to Outlands, and saw the linked overmasters and slaves, I had a sinking feeling that I was going to have to kill some of them to complete my quests. But I was pleasantly surprised to see that the slaves run away after you kill the overmaster, allowing you to complete the quests without killing the slaves. It even adds a bit more challenge to the affair, as you can't use reactive damage (Holy Shield, Ret Aura, etc.) safely.
It's a small touch, but it made me happy. In many ways, Blizzard excels at these small effects, and its a major reason I love their games.