The major departure from the graphic novel is the creation of a "Gorgo in Sparta" subplot which does not take much screen time but envelops the main story and makes the final and anticipated catharsis more rewarding. The historical Gorgo would have known that she would not see Leonidas again; his last words to her are said to be "to marry a good man and have good children".
Many of the lines in the film can be traced to the ancient sources either directly or indirectly. For example, "tonight we dine in hell" is found verbatim in Plutarch who records that Leonidas urged his men to "have breakfast for tonight we dine in Hades" (αριστάτε ως εν άιδου δειπνήσομεν). Gorgo manages to fit in two classic lines, "this shield or on it" (η ταν η επι τας) which was generally attributed to Spartan women, as well as her own "because only Spartan women bear men", which was, however given not to Persian envoys but to an Athenian woman.
Leonidas' exhortation to "come and get them" (μολών λαβέ) when asked for his weapons is preserved, as is Dienekes' wisecrack about fighting in the shade, followed by a stunning visual display of how Persian arrows could obscure the sun. So is the Simonidean epigram, "Stranger go tell the Spartans that here obedient to their words we lie" (ω ξειν αγγέλλειν λακεδαιμονίοις ότι τήδε κήμεθα τοις κείνων ρήμασι πειθόμενοι).
Leonidas' line on "we brought more soldiers" was actually uttered by King Agesilaus:
He gave orders that all the allies, of whatever country, should sit down promiscuously on one side, and all the Lacedaemonians on the other: which being done, he commanded a herald to proclaim, that all the potters of both divisions should stand out; then all the blacksmiths; then all the masons; next the carpenters; and so he went through all the handicrafts. By this time almost all the allies were risen, but of the Lacedaemonians not a man, they being by law forbidden to learn any mechanical business; and now Agesilaus laughed and said, "You see my friends, how many more soldiers we send out than you do."
The festival of the Carneia did in fact result in the Spartans missing out on the Battle of Marathon; they marched from Sparta after the festival was over and arrived after the Athenians had defeated the Persians. This festival was also the reason why only Leonidas and his 300 were dispatched to Thermopylae. There is no mention in the ancient sources of Spartan treachery in this particular instance, either in Sparta itself or by Ephialtes, who in reality was not a Spartan.
The Spartans did consult the oracle at Delphi, who was however usually an old woman rather than the most beautiful young one. It is said that on account of this oracle Leonidas marched to his death, for it was said that Sparta would be destroyed or lose a king. It is ironic that -in accordance to modern sensibilities- Leonidas' obedience to the oracle has been re-interpreted in this version as part of a struggle against mysticism. This is a minor quibble though, and not at all distracting as the "Rome was founded as a Republic" nonsense of Gladiator.
In reality, the Spartans would have been fully armed, and long-haired. The long hair made them look more terrible, but also served a practical purpose of cushioning the head as it was used as "padding" within the helmet. They would all wear their red capes -which made blood more difficult to see and fostered courage- while officers would have transverse horse plumes, their own distinguishing mark. They would also have beards but no moustaches; the ephors fined those who grew a moustache.
It is understandable that scenes of Spartans single-handedly and out of rank slaughtering many enemies were included in the movie, but as King Demaratus said to Darius, Sparta's strength lay not in the enormous power of its individual men, who could be matched by other men, but in their unity. This strength of the phalanx is shown nicely in the first battle scene and explained laconically in Leonidas' speech to Ephialtes.
Many "progressive" critics have complained about the lack of any homosexual hints in this movie. Contrary to modern belief, and according to the best ancient sources, the Spartans abhorred homosexuality; practicing homosexuals as well as effeminate men suffered a fate in Sparta that would be little envied by the modern "gay" movement. The Spartans did encourange chaste pederasty of a non-sexual nature; unlike modern societies where young people fraternize only with their peers, in the context of ancient warfare, bonds other than familial ones had to be fostered, since the phalanx included young and old alike.
The 300 is not the best movie about Thermopylae that could be done, but it is certainly a fine effort. Let's hope that someone will be inspired to dramatize other tales from the Persian Wars in the future.