It's not an historical movie... It's based of a historical event that is more mythical than real. Yes the Spartans fought at a large disadvantage and yes they all died there. It wasn't about making an accurate historical movie. There's books and documentaries for that, they wanted to bring epic to the general public.
And in term of epicness, there's nothing better in the last 100 years of movies.
Oh and you're missing quite a few historically accurate things like the way they fight (Phalanx), the spear, the shield and the xiphos are accurate in the movie. Also, you're stating that it's much less than the real life movie, yet herodotus himself estimated it at around the same number in the movie. Herodotus is the where the legends starts. It's actually based around that more than the actual true scientific modern days evidence. (5k vs 100k if I remember correctly).
Then there's the traitor after the first day of battle, thats true too. (Albeit it's not a mutant me thinks) Then there's the part about the 300 spartans (and a few hundred other greeks) that held the last stand which is also believed to be true.
Then there's the Immortals which are also a real-life force.
I tell you this movie is a lot deeper than you would think