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 <i>correct</i> about the movie than to list the differences.