Utopia sounds like a fascinating place. Not a Paradise, but more a society that functions better than the one we find ourselves living in, or the one that More did back then. It can be seen as a Dreamworld, but some of it could work in reality.
I agree with you, any society that allows slavery hardly sounds like an Earthly Heaven to me! However from our study of the Greeks and Romans, and now medieval times, it seems that people in the old days didn’t have a moral objection to slavery. The writings of 19th Century Abolitionists may be why we do.
I should also mention that the opposite to utopian – an ideal world – is dystopian, which usually means a nightmarish society. Modern dystopian literature includes 1984, Brave New World, Fahrenheit 451, The Hunger Games and the Matrix series of movies.