Well, the thing that always annoys me is that everyone assumes slavery always has to do with race. It doesn't. Most slavery, historically, was conducted against people of the same race. It's just American History that people don't see beyond.
Slavery is a social construct.
It's a status where you can basically abuse people of a lowest possible social status without fear of immediate consequences. But nobody wants to see the people they know or care about treated like that, so, socially, they construct some reason for why those other guys deserve it. That's what separates the kinds of slavery, the reasons why people find it justified...
First, the most common was debt slavery. Debt slaves were people who either couldn't pay their loans or their taxes, or the children of those people (because children were taken as collateral on a loan or taxes). It was "fair" because "they should have paid their debt/taxes", even if the circumstances were beyond the control of the debtor in the first place (bandit raids burned the fields so there was no grain to pay the taxes). Ruthless nobles could even purposefully set taxes high enough to drive all their subjects into slavery (but risk revolt). It requires the mindset that the people at the bottom classes of society deserve to be there because they got there of their own fault.
Second, there was war slavery. Vae Victus, or at least, "Might Makes Right" has to be believed for a society to accept this kind of slavery. They were being punished for being on the losing side of a war, and so the victor could do what they wanted with them. Notably, the children of war slaves were freed, as were many war slaves themselves after some years of service. Roman war slaves could often become full Roman citizens. On the other hand, it was war slaves that were sold to become racial slaves for the colonies of Europe, including America. But the notion that Might Makes Right is required to sustain this - without believing that violence is the answer, how can you justify treating someone that way just for losing a war?
Finally, there's the racial slavery. This is the type of slavery that requires people to believe that it's fine to abuse this other race but not mine because obviously my race is superior. It's a type of slavery that requires all practitioners to be racist. There's a wonderful quote I have on the nature of slavery from a preacher before the American Civil War - "We must believe that the black man is less than human, for if we were to believe otherwise, then we would be forced to conclude we are less than Christian." The rationalization is that "obviously, they're better off being slaves, because they're such savages without us directing every moment of their lives".
There are also always severe negative consequences for slavery: Many labor-saving devices were invented by Egyptians and Greeks through even Medieval Europe that were discarded because if they made work more efficient, what would they do with the slaves? It was only after the Bubonic Plague came through and forced people to start taking up labor-saving devices like printing presses that Europe advanced out of the dark ages.
It's just so frustrating that I have to keep finding new ways to say "Slavery means more than just making people do what you say or you'll hit them."