From what I've heard, the enlightenment played into making American slavery as bad as it was. Since the enlightenment said that it was bad to enslave people, but slavery was very profitable, a pattern of rationalizations were built up that Africans were innately inferior and slavery was good for them.
I wanted to bring attention to two posts from Razib Khan's Discover magazine gene expression blog (some of you may have been readers of the still active original gnxp) on the polemic surrounding Pinker's The Better Angels of Our Nature.
Relative Angels and absolute Demons (and the related But peace does reign! )
I generally agree with some of his arguments, but found this quote especially as summing up some of my own sentiments: