The word spread very quickly. Even months before the release, millions of people knew about the Spartan King Leonidas and 300 Spartans fight to the last man against Persian King Xerxes and his army of over one million soldiers. Check out the number of views for the first trailer (2,778,451) and the second trailer (2,238,651)! Soon you couldn't see a single site without an image or comment related to the film. Ytmnd even made an interesting retro version of 300 - 8-bit NES style!
What's more, the soundtrack composed by Tyler Bates not only has given rise to some controversy in the film composer community due to its striking similarity to several other recent soundtracks (see "Copyright Issues" on Wikipedia page for more information), but also conquered everyone's heart. Most importantly, the song used in the trailers of the movie, Nine Inch Nails's Just Like You Imagined (absent from the soundtrack), adds epicness to almost anything. Take a look at these videos after the jump.
[ Warning: Videos contain violence, fun... etc, etc. Prepare for glory! ]
Footage of a real fight
South Park spoof
Star Wars spoof
Halo 3 spoof
World of Warcraft spoof
Perhaps it's King Leonidas' charisma