Accidental Aesop: The film adaptation of 300 is often interpreted to glorify secular, westernized countries standing against the religious extremism and intolerance of the Middle East. However, some critics pointed out that in the film, Persia is a massive, wealthy and culturally diverse empire bent on expanding its influence throughout the world, while the Spartans are a small group of dedicated, zealous fighters who are willing to break the rules of war and martyr themselves to resist the invaders, which makes Persia represent the United States and Spartans represent the terrorists instead.
Awesome Art: The comic book contains some of the best art Frank Miller ever drew. The coloring also falls under this.
Designated Heroes: Ordinarily, you wouldn't be rooting for a side that glorifies warfare, practices eugenics, is profoundly tribal/racist, kills diplomats and systematically kills wounded and those attempting to surrender. The film gets away with it through sheer Black and Grey Morality, as the Persians are portrayed as even nastier tyrants who employ literal monsters, enslave entire cultures and massacre villages on their path, but this does not erase the Spartans's own crimes by modern standards (and cannot be accomplished without a healthy dose of Artistic License History, as those who are more familiar with the historical Greco-Persian Wars will know). As a consequence, 300 can be perceived to make an awkward stance by painting the Spartans as morally relatable even though it doesn't bother to cover most of their barbarism.
Designated Villains: The Persian soldiers are repeatedly mentioned to be nothing more than slaves forced by their fear of the king to fight. This is even meant to be derisive, as it implies they are at fault for not having enough bravery, strenght or virtue to rebel against Xerxes or die trying as the Greeks do. Naturally, it doesn't make them any less sympathetic, especially to modern sensibilities. (Also because, as said above, this happens to be a huge historical deviation: Persian soldiers were certainly citizens forced into service by their monarchy, but they weren't literal slaves, while the Greeks didn't shy away from slavery themselves and had no problem with it.)
Gratuitous Special Effects: This movie makes heavy use of prosthetics, Green Screen and lots of CGI. The same battle was depicted in the movie The Three Hundred Spartans decades earlier with little more than fancy costumes and prop swords. The comic is comparatively more realistic with its visuals.
An in-story example: "We will fight in the shade."
Also, the movie did that to The Man Who Saves the World due to Leonidas having a very similar appearance to the main villain in that film.
It's hard to take many previous works with the phrase "This is madness!" seriously anymore due to the 300 film.
Leonidas isn't the only Spartan with an affinity for making gods bleed. Then again, he's not the only character from a Zack Snyder movie to do so either.
Leonidas' most well-remembered line "This Is Sparta!" is this when you realize that another warrior character played by Gerard Butler has a son who's known for starting the movie with "This is Berk."
Leonidas's line about dining in hell becomes this if you play Fate/Grand Order, since that game's incarnation of Leonidas is heavily influenced by this film, yet is terrified of ghosts - which hell would be populated with.
Ho Yay: This movie might have been called Ho Yay: The Motion Picture. In fact, real life Spartans in the agoge were encouraged to have a relationship with an older master who will train them. In some cases, girls had to shave themselves bald so that they could look like boys and get married.
While the Spartans were already commonly regarded by historians as one of the most badass civilizations in all of history, this movie, also making bored high school students interested in a quaint little city state that they would not even know existed in the first place, exaggerated the idea further that the Spartans were really a race of hypermuscular Supermen who can each kill millions by themselves while wearing only underwear. It's like applying Chuck Norris Facts to an entire ancient city.
Spartans don't need armour. Their abs are harder.
Memetic Molester: Xerxes. The Spartans also suffer from this as well due to all the massive Ho Yay Fanservice they provide, and the historical fact that Spartans tolerated active homosexuality and pederasty while living in communal barracks.
"Xerxes is a Goa'uld" quickly gained lots of traction, thanks to his unnaturally-deep voice which even sounds like it uses the same audio effects as the System Lords did and god complex. All he's missing is the glowing eyes.
Moral Event Horizon: If Theron didn't cross it by using Persian money to bribe the ephors into warning Leonidas against going to war against Persia during an imminent religious holiday, he definitely did so by raping Gorgo and attempting to out her as an adulteress at the Senate meeting the next day.
We could also call this Narm: The Motion Picture, what with every single actor on a 100% scenery diet.
The sheer premise: muscled badasses taking on a technologically and numerically superior continental empire of a million nations (with MONSTERS and Ninja Orc Supersoldiers) while wearing only briefs. If that's not Testosterone Poisoning, nothing else is.
The Persian executioner, being an ogre-like type with blades instead of arms, looks like he came out directly from either Diablo or Warcraft.
A number of lines from the comics and the movie are actually from Herodotus, including "fight in the shade" and "Tonight, we dine in Hades" (Hades is short for "Hades's kingdom", the underworld, which was where all afterlives were, for Ancient Greeks). The Spartan epitaph planted by the side of the road is actually still visible as a marker from the Classical period:
Go tell the Spartans, passer by, That here, by Spartan law, we lie.
Queen Gorgo also apparently said "Only Spartan women give birth to real men." Though this is believed to have been said to another Greek, not to a Persian messenger.
The Spartan Way: the real life Spartans literally made this trope. However, real life Spartans didn't go to war naked; they were portrayed that way because in ancient Greece, muscular nakedness was a symbol of heroism.
Ron the Death Eater: The movie Spartans get hit by this heavily, with the actions of the real Spartans used to attack them due to Values Dissonance (what with things like, for instance, violence against children being far more of a taboo in most of the modern world than it was in most ancient societies, and also the concept of eugenics having very specific connotations nowadays) and all of their actions painted in the worst light possible.
Strawman Has a Point: When the Persian herald confronts the Greeks building a wall from dead bodies and screams that they are barbarians, it's hard to argue that he isn't somewhat justified in his horror and rage.
The finale: the camera pans across the horizon and shows that the sacrifice of Leonidas and his brave 300 has inspired thirty thousand Greeks to fight against tyranny.
The final scenes of the movie, though thanks to Dilios's verbal eloquence and expertise for Rousing Speeches it leaves a bit of a hope spot for the Greeks (See Quotes page for the full speech.)
Another scene that deserves mention is the one where Leonidas is leaving Queen Gorgo for the last time. As Dilios narrates with solemn dignity: "Goodbye my love. He doesn't say it. There's no room for softness, not in Sparta. Only the hard and strong may call themselves Spartans. Only the hard. Only the strong."
And that tearjerker inspires another tearjerker near the end, when Leonidas, peppered with arrows, the only Spartan left standing as his comrades die around him, raises himself up, and declares his love for Gorgo just before the final rain of arrows fall.
Suspiciously Similar Song: Most of Tyler Bates's score were derived from other works such as Titus, to the point that Warner Bros. not only apologized and resolved their mistake, but they also posted a note on later releases that the music was "derived from preexisting compositions not authored by Tyler Bates."
Complicated again, as the film was made during The War On Terror, which Frank Miller supports, but the original comic was written a decade earlier. This has lead to such a bad Misaimed Fandom that a March 2007 press conference saw director Zack Snyder asked by a reporter whether King Leonidas was meant to be George W. Bush or Osama bin Laden. Original author Frank Miller claims that his comic to a large degree was inspired by the 1962 film The 300 Spartans, which is often considered to be a metaphor for the Cold War. Whether such a message was intended or not is far from clear.
To further complicate things, people have made arguments for the movie taking both sides as an allegory for War on Terror: the Spartans can be seen as representing a small local native people being threatened by an enormous expansive imperial force, and white westerners boldly slaughtering dehumanized brown-skinned middle easterners.