The problem with this movie is simply that it exists. There is nothing particularly wrong with the film. It is just not as good as it could've been if it wasn't separated for its pair. A tactic that has been successful with other films and narratives in the past but did most certainly did not work this time.
The Story
This movie finishes the war and finishes the series. I'm not telling you anything else.
The Stars
The Message
Overall
Let's look at the times this tactic has worked in the past. Harry Potter's final book was split into two films. This works because we have one location for the final movie: Hogwarts. It is a natural break in the story: our three heroes decide to fight instead of hide. Twilight's final books was split into two films. This works because our hero has "awakened" as a new creature. The world is entirely different to vampire Bella than it was to human Bella. So new film techniques can be used and even the 3D makes sense here. When it wouldn't have worked in the previous film.
This time, it doesn't work. There is no natural break in the story. There is no character awakening. The location is the same as the last movie. By spliting this book is half, you lose all of the momentum the last part had created. Now, we don't remember who the new characters are, nor do we really care when they die in the beginning. The character development is not as clear or as well crafted than in the book because you can't remember the nuances of the character in the previous film. Nor do you really care now to go back.
The new movie created more money for an opening night and more money from the people who forgot to see the previous film. It does nothing for the story. It does nothing for the characters. It is simply to make more money. And not even more money for the franchise as a whole because this movie wasn't very good, newcomers won't want to go back and watch the old movies.
I hope this movie will stop this trend of splitting films in half. It just doesn't work.