Re Brin on 300: If Brin's impression of Sparta was actually correct, the real question is why Sparta wasn't like most oppressive states where the ruling classes easily accepted Persian suzerainty in exchange for support of their privileges. And if his impression of Athens was correct, the real question is where the Athenian Empire came from.
Re 300: Imagining the ephors as a hideous rabble and Xerxes as a transvestite freak and Persians as demonic says everything you need to know.
Re Frank Miller completely losing the plot: How do we know he ever had it. From what I see of the Nolan version of Batman, which is apparently a blend of Miller and Insomnia, there's no reason to think Miller ever had the plot.