The movie is basically "guy gets cast as Messiah by evil cabal machinations and is too big a baby to do anything about it". The end.
Leaving aside for a moment the sheer complexity of the themes and the plot and the universe in the book —that didn't make it through— the movie doesn't even stay faithful to itself. Every single person who's had any influence on Paul gets discarded just so he can fulfill his ultimate destiny of being a sad, wet blanket with a "welp, I guess we're doing that" attitude.
But seriously, how do you manage to make two movies and have nothing important from the rich Dune universe make it through? This could have just as easily been set in the Star Wars universe with only minor alterations and nobody among the general public would have batted an eye.