The Persian army wasn't composed of monsters, to start. In the movie, many of the enemy soldiers are deliberately made to be monstrous and non-human.
Additionally, the movie pays only lip service to the thousands of non-Spartan soldiers who fought (and died) there.
The battle scenes are complete fabrications - the Spartans pay lip service to their proper formation, but abandon it for individual heroics at the earliest opportunity.
Essentially, it would be easier to ask what is correct about the movie than to list the differences.