For fans of the book, this movie actually holds true to the plot written by Stephanie Meyer. You'll even get to see scenes surrounding Jacob the way Meyer wrote them in the book. While there are those cheesy lines and sappy scenes (as it is with most teen flicks), those that haven't read the books will be able to follow the plot and enjoy the movie for what it is: a guilty pleasure.
There were some slight differences and here is my famous list for you:
1. Rosalie's reason for helping Bella: The movie didn't explain why Rose was so bent on helping Bella and the baby. It's no secret that Rose has never really liked Bella and her reasoning was just another display of this. Her actions after the baby is born shows this a bit, but it's not really all that clear.
2. Bella's need to be near Jacob: We're all familiar with the triangle between Bella and the two boys, but during her pregnancy, Bella finds herself needing Jacob around more so than ever. It's not until after Jacob imprints that we realize it was because the baby wanted him around. They also took out the alternating hot/cold aspect that Jacob and Edward did to keep Bella comfortable.
3. Jacob's freak out and need to imprint: In the books, Jacob has a moment where he needs to escape and go some where. Edward gives him the keys to the car he got Bella (another aspect not seen in the movie) and he goes out for a drive. Jacob, feeling desperate, tries to imprint on someone with no success.
What's nice is that I understand why they took out these aspects of the books. I have to say, it was definitely the better of the adaptations thus far. The last one comes out next November. Let's see how it goes.