While slaves were indeed drawn sometimes from foreign peoples, either captured in war with the Romans or sold to them, in most cases they were Romans themselves or at least from Italy. The Romans did not base slavery on race, and happily enslaved their own. Furthermore, the majority of slaves that weren't Italian were Greek, who looked similar enough to them to make it hard to distinguish. As a consequence, the percentage of non-Roman and non-white slaves in the show is much higher than reality (and watch out also for this whiteness - Roman characters are played by actors that in many cases are fairer than most Romans were, vice versa for Gauls and so on).
There is no mention of the fact that high-level slaves (such as household ones and gladiators) often earned their freedom, or that slaves commonly were allowed to, which instead gets treated as exceptional.
While runaways were sometimes killed, usually the punishment was "just" branding and/or a flogging. After all, slaves were valued as property, if nothing else, and losing one was still an important economic loss.
In the slave market scenes, the prices mentioned are a fraction of what any slave - let alone a gladiator - would actually have cost at the time. Since understating the prices doesn't help the story, this is probably a genuine mistake.
Illithyia buys a gladiator for herself when her father is still alive. Patria potestus stated that a son or daughter could only own property once their father died. She also receives an allowance from her husband, but this was also forbidden by law; a man couldn't give any gifts to his wife, including allowances. Besides, with her father still alive, anything given to her would legally belong to him.
One scene features a party in Capua where slaves are painted white to look like statues, emphasizing their status as things. In reality, Roman statues were brightly painted, but that paint faded long ago and now modern audiences are only familiar with Roman statues as white.
Every character played by an actor of sub-saharan African descent is referred to as a "Numidian", who were an olive-skinned Berber people from North Africa (think modern day Algerians). It is possible the writers mistook Numidians for Nubians, who actually are black Africans. Further, the term for black people generically was "Ethiopian", and likely would be used instead.
The pilot episode portrays the Getae people as inhuman savages in comparison to the noble Thracians. In reality, the Getae were so similar to the Thracians that historians are still a little unsure what the difference was.
Spartacus refers to himself as a Thracian with obvious national pride. That was a Greek term used to refer to his people, and there's no evidence that they called themselves that.
Tiberius' motivation for raping Caesar and the latter's silence about it afterwards only make sense in a modern context. In Ancient Rome, the rape of any free-born citizen held the death penalty - and while the rape of an adult male wouldn't be directly punished through courtsnote Unlike the rape of a maiden, matron or any sexual contact between a man and a citizen boy who wasn't yet declared an adult, which usually happened at 15., the victim would be full within his rights to kill his attacker.note We have a historical case where a soldier got a medal for killing his superior officer while the latter tried to sexually assault him. And even if it came to a political scandal over this incident: Caesar is the pater familias of an old, respected (though somewhat impoverished) noble house, with a young daughter to ally himself to another by engagementnote He eventually marries her to Pompey, to cement the Triumvirate involving him, Pompey and Crassus., and has already served his country as a priest, minor official and by wiping out some pirates, whereas Tiberius is the barely adult, completely undistinguished son of nouveau riche Crassus, whom the entire Senate hates for not "knowing his place". Who do you think they'd side with in a legal battle? Also, aside from signing his own death warrant and dooming his family, Tiberius's threat of making the humilation public and thus harming Caesar's reputation doesn't make sense with this particular victim, as the real Caesar was rumored to have had an affair as a teenager with King Nicomedes of Bithynia while on a diplomatic mission (most historians doubt the rumor and think it was most likely concocted by his rivals in an attempt to discredit him—or at least piss him off, but you never know), leading his enemies to quip that he was "a man to any woman, and a woman to any man".note Which, despite general expectations that Roman men should want to fuck anyone, was a genuine problem, as proof of voluntary bottoming - i.e. acting "like a woman" - would have saddled the man so accused with the legal status of "infamy", which among other losses in civil rights, would prevent him from ever holding public office. Since Caesar would have been the younger and lower-ranking partner in that affair, the Romans automatically assumed that he would have bottomed, because in their society, the person of lower social rank always was supposed to "serve the pleasure" of the higher-ranking one. Free-born male citizens couldn't have a sexual relationship with each other because they were of equal social rank. Friends and enemies alike happily repeated the rumors throughout Caesar's life, much to his annoyance, but they still never seriously harmed his career or stopped him winning the respect of his legions. It also doesn't fit with what we know of the historical Caesar's personality: badass who was personally brave and supremely self-confident, a man who after being kidnapped and ransomed by pirates (after first doubling his own ransom), hunted them all down and crucified them, and elsewhere also received the very rare honor of the corona civica (civic crown), awarded for saving the life of a fellow citizen.
Action
The type of Roman armor known today as lorica segmentata (segmented plate armor) appears in the show, but it was invented much later in the days of the Roman Empire. It is, however, the most easily recognisable Roman armour, so chalk this one up to Small Reference Pools (along with the over-representation - and occasional misapplication! - of their famous testudo formation).
Gladiator matches were usually not to the death, except if against common criminals (as shown). Instead they were largely to first blood. Only if a gladiator made a particularly poor showing would he usually be killed. Gladiators were of very high value, with years of money and effort put into their training, so killing them off this casually would be a huge waste, if nothing else.
Further, gladiators in reality were much beefier than most shown in order to withstand cuts and draw blood with no serious injury. The actors on the show went to extreme lengths to maintain lean, shredded physiques to fit modern aesthetics.
Events
Several events in the show differ from the historical record. The initial escape was carried out by seizing kitchen utensils, Varinius survived the war and didn't even field troops until after Glaber's defeat, and Crassus wasn't given command until after the split between Spartacus and Crixus, and Crixus' death.
Spartacus is believed to have spent one year at the ludus in reality, while it's two in the show. Also, there's no historical record to suggest he was a champion gladiator.
While little is known about Spartacus's wife, beyond her being a prophetess from the same tribe, historical record suggests that she was actually enslaved with him, helped in the uprising, and survived at least as long as he did.
In War of the Damned, the movements of Spartacus' army are essentially reversed from the historical record. note Historically he moved north, historians believe to escape over the Alps before turning south after the split with Crixus and Crixus' resulting death (historians are unsure as to why, exactly, but revenge is a popular theory), eventually moving to the southern tip of the Italian penensula. Records indicate this was with the intention of sailing to Sicily, and he was then boxed in between Crassus and the sea. In the show, Spartacus moves south first, and his intentions towards Sicily are for strategic purposes, not escape, before fleeing north and being caught between Crassus and the Alps.
While Spartacus did attempt to fight his way to Crassus during his final battle, he never made it, and did not have a Duel to the Death. Crassus was also not known for personal combat prowess.
Caesar was not involved with putting down Spartacus' revolt in any capacity.
