Based on Frank Miller's graphic novel, "300" is very loosely based the 480 B.C. Battle of Thermopylae, where the King of Sparta led his army against the advancing Persians; the battle is said to have inspired all of Greece to band together against the Persians, and helped usher in the world's first democracy.
Here are some ideas:
It is not an objective critic of the movie, but rather a description of what you felt watching the movie.
You should not hesitate to write more about your emotions than about the movie itself.
And take care not to divulgue any information about the plot!
But in the end, other users are the one rating the comments, so you may do as you feel it should be done.