For those asking for definitions, I'll use a quote from a book I just read: "Slavery is the ultimate form of being ripped from one's context, and thus from all the social relationships that make one a human being."
(of course, in fantasy settings, dwarves and elves are also supposed to be of equal personhood)
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Just a few general facts:
*Revolts concerning relationships of pure hierarchy (slavery and caste systems) are historically less common than ones based on things like debt peonage where people are "supposed" to be equals, but due to shenanigans, aren't. And the big world religions seem much more concerned about debt bondage and usury than caste/slavery. (hence strong restrictions on usury, regular debt cancellations; but often little opposition to unarguably worse forms of subjugation) Presumably this says something about the way relationships of hierarchy are based on precedent, not logic or principle.
*When slave and caste revolts do occur, it is often about their treatment, not the principle in question.
*During times when chattel slavery was common, people viewed it the way most modern people do war "oh, it's terrible, but it's unrealistic to think of a world without it". Ironic, considering there's several times in history where is is abolished.
*IRL, widespread use of precious coin only occurs in eras of mass war and slavery. (in other times, money is based on personal or state credit) I just figured I'd throw that in there because most GMs assume everyone in the world is using gold and silver coins. Adventurers are shady people, who would let them run up a tab? ;P
*one often becomes a slave in situations where one would have otherwise died (for example, one would only be enslaved for not paying a debt if it was OK to kill an insolvent debtor in that culture). eras where mass slavery was common treat someone enslaved as legally dead.
*roman slavery was weird. 1. It wasn't based in any way on bigotry, anyone could be kidnapped, fall into debt that can't be repaid, lose a battle, and become a slave. And yet, once freed, a slave was a free citizen. 2. The master had absolute property rights over the slave. Unlike much of the ancient world, in one's household, you could do /anything/ with one, no matter how vicious. This results in weird things like: people intentionally selling themselves into slavery to become a citizen (with a friend holding the money to buy the freedom back), but during this time their Dominus could cut off fingers or murder them for no reason.
*"war captives" is always seen as the most socially acceptable reason for slavery. So much so, that slave-traders will often claim most of the people they are transporting were captured in war, even if most of them were really: sold by their parents to pay off debt, randomly kidnapped, made slaves as punishment for alleged minor crimes by local authorities, were actually someone else's pawn/servant before they were transported etc. Often times "war captives" becomes this big legal fiction people in Empires tell themselves to make them feel better.
