For one thing, slavery went out of style during the Dark Ages. Romans had notoriously loved them some slaves, but improvements in farming technology and better-bred draft animals made forced human labor less necessary as time progressed. Instead, the classes that would probably have found themselves in slavery were mostly either free workers or, at worst, serfs. The latter were still technically not free (they couldn't leave the land without their lord's permission), but enjoyed a much greater freedom than slaves.
The rise of Christianity, while admittedly resulting in a lot of people being set on fire, also saw a dramatic increase in charities. Almost immediately after the church gained a foothold in Europe, they started introducing a widespread system of charity that distributed food, clothing, and money to those in need. Perhaps not by coincidence, the concepts of goodwill hospices, hospitals, and shelters for the poor were also invented during the "dark" ages, paving the way for the public health care system.
And, eventually, a health care system based on something besides incense and the Virgin Mary.