I've never gotten an impression that the general public has any particular notion of the Romans being "enlightened" with respect to slavery. Most people seem to know only that there were slaves, but have no idea how large the slave population was. People today are eager, almost desperate, to scold somebody for being horrible, but they don't think beyond that point. To me the more significant thing by far is the sheer volume of slavery, and that Roman society was only able to exist because of it.