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.
edit:
I've found an interesting PDF which discusses the differences (link is to actual PDF). There are important differences in Spartan motivation, the reason such a small force was sent, Persian army size, chronology (especially the well incident), and equipment.
Cracked says the movie was saved by its inaccuracies.
This page takes my advice (in advance) and lists the accuracies instead.