Ahhh ... Live Science gets closer to what I was looking for (cf the thing from Payvand below) ... written by Ephraim Lytle, assistant professor of Hellenistic history at the UofT (this was originally in the Toronto Star, but I never found it online):

History is altered all the time. What matters is how and why. Thus I see no reason to quibble over the absence in 300 of breastplates or modest thigh-length tunics. I can see the graphic necessity of sculpted stomachs and three hundred Spartan-sized packages bulging in spandex thongs. On the other hand, the ways in which 300 selectively idealizes Spartan society are problematic, even disturbing.

We know little of King Leonidas, so creating a fictitious backstory for him is understandable. Spartan children were, indeed, taken from their mothers and given a martial education called the agoge. They were indeed toughened by beatings and dispatched into the countryside, forced to walk shoeless in winter and sleep uncovered on the ground. But future kings were exempt.

And had Leonidas undergone the agoge, he would have come of age not by slaying a wolf, but by murdering unarmed helots in a rite known as the Crypteia. These helots were the Greeks indigenous to Lakonia and Messenia, reduced to slavery by the tiny fraction of the population enjoying Spartan "freedom." By living off estates worked by helots, the Spartans could afford to be professional soldiers, although really they had no choice: securing a brutal apartheid state is a full-time job, to which end the Ephors were required to ritually declare war on the helots.

Elected annually, the five Ephors were Sparta's highest officials, their powers checking those of the dual kings. There is no evidence they opposed Leonidas' campaign, despite 300's subplot of Leonidas pursuing an illegal war to serve a higher good. For adolescents ready to graduate from the graphic novel to Ayn Rand, or vice-versa, the historical Leonidas would never suffice. They require a superman. And in the interests of portentous contrasts between good and evil, 300's Ephors are not only lecherous and corrupt, but also geriatric lepers.

Ephialtes, who betrays the Greeks, is likewise changed from a local Malian of sound body into a Spartan outcast, a grotesquely disfigured troll who by Spartan custom should have been left exposed as an infant to die. Leonidas points out that his hunched back means Ephialtes cannot lift his shield high enough to fight in the phalanx. This is a transparent defence of Spartan eugenics, and laughably convenient given that infanticide could as easily have been precipitated by an ill-omened birthmark.

300's Persians are ahistorical monsters and freaks. Xerxes is eight feet tall, clad chiefly in body piercings and garishly made up, but not disfigured. No need – it is strongly implied Xerxes is homosexual which, in the moral universe of 300, qualifies him for special freakhood. This is ironic given that pederasty was an obligatory part of a Spartan's education. This was a frequent target of Athenian comedy, wherein the verb "to Spartanize" meant "to bugger." In 300, Greek pederasty is, naturally, Athenian.

This touches on 300's most noteworthy abuse of history: the Persians are turned into monsters, but the non-Spartan Greeks are simply all too human. According to Herodotus, Leonidas led an army of perhaps 7,000 Greeks. These Greeks took turns rotating to the front of the phalanx stationed at Thermoplyae where, fighting in disciplined hoplite fashion, they held the narrow pass for two days. All told, some 4,000 Greeks perished there. In 300 the fighting is not in the hoplite fashion, and the Spartans do all of it, except for a brief interlude in which Leonidas allows a handful of untrained Greeks to taste the action, and they make a hash of it. When it becomes apparent they are surrounded, this contingent flees. In Herodotus' time there were various accounts of what transpired, but we know 700 hoplites from Thespiae remained, fighting beside the Spartans, they, too, dying to the last man.

No mention is made in 300 of the fact that at the same time a vastly outnumbered fleet led by Athenians was holding off the Persians in the straits adjacent to Thermopylae, or that Athenians would soon save all of Greece by destroying the Persian fleet at Salamis. This would wreck 300's vision, in which Greek ideals are selectively embodied in their only worthy champions, the Spartans.

This moral universe would have appeared as bizarre to ancient Greeks as it does to modern historians. Most Greeks would have traded their homes in Athens for hovels in Sparta about as willingly as I would trade my apartment in Toronto for a condo in Pyongyang.