The story oftentimes made no sense to me or left me confused. Since I've never read the book, I'm guessing that's why. The screenplay could have used more polish. Dialogues though, were perfectly authentic.
Ermm...its not really a difficult plot is it ? There's some treasure, they all want it. The goodies get it, the baddies don't. Have I missed anything ? Maybe read the book ...the dialgoue came in large chunks from it which is probably why it was so authentic as you say.
The constant switching of loyalties, vague motivations for some of the characters' actions and a few things that happen off screen would be my examples of moments which were a bit confusing to me. Overall, the story is very simple to follow of course.