The cast was ridiculously stacked
Admittedly, it's a tad ridiculous to think about these obscenely buff, barely clothed men shouting at one another in a Canadian storage unit. At the time, the cast were a lot of unknowns -- but watch again, and you'll find that one of the most maniacal warriors with the tiniest of loincloths is none other than two-time Academy Award nominee Michael Fassbender. (He's the one who, along with King Leonidas, pulls the Hellenic version of the ol' Statue of Liberty Play to kill one of Xerxes' generals.)
King Leonidas, of course, was played by Gerard Butler, whose career path hasn't been quite as noble, but at least he keeps busy. Right before 300 he was in The Phantom of the Opera, which means he was very accustomed to shouting. The sleazy politician was played by Dominic West, thus making 300 the first in a very long series of films where audiences would turn to each other and say, "Wait, Jimmy McNulty is ENGLISH?!" And Lena Headey first caught mass attention as Queen Gorgo; since then, she's appeared in no shortage of fanboys' regal fantasies as Cersei Lannister on Game of Thrones. We're not saying all these actors owe their whole careers to 300, but what other movie would have let them strut their stuff in such a provocative, half-naked manner at a such a clutch moment in their inchoate careers?