It's no Citizen Kane - but that doesn't make it brainless
This is one of those few movies that I watch purely to have fun. I saw it for the first time back when it came out, and it has since provided many recurring jokes in my household (there was a time for an entire summer when "This is Sparta!" was said probably twice a day, complete with my brother kicking me in the chest). It's extremely stylized - perhaps to a fault - and there's more action and slow-mo than any other film I've seen about ANCIENT GREECE, for God's sake. But that being said, it's actually a lot smarter than most people make it out to be. I re-watched it the day after I took an exam on Greece, and the film is surprisingly historically accurate. The warrior society, how Sparta treated their women, and the battle of Thermopylae are all portrayed very well. While they do take quite a few other liberties - most egregiously, making Persians black - a lot of other things are very faithful and insightful. The movie could have made the climax a triumphant, historically-inaccurate Spartan win, but instead they kept it faithful to history and let Leonidas die, using the drive for vengeance as the catalyst for Sparta amassing a larger army. It made the movie much more effective, emotionally and historically. The film, while it does have its overly-manly, sometimes homoerotic tendencies, is still an insanely fun experience. Its scenes are gorgeous, and it made Leonidas into such a badass that he's now about as well-known as Alexander the Great. Don't let the many ripoffs (like the bland "Immortals") make you think any differently: 300 was a unique and fantastic film that is much smarter than its visuals lead you to believe.
Gentlemen, you can't fight in here! This is the war room!