I found this article on Twifans and it's really interesting!
Breaking Dawn is very different from the first three novels in the series (Twilight, New Moon, and Eclipse) and saw numerous mixed reviews during its release week. The millions-strong fan base was divided — everyone either loved it or hated it.
We’ve come up with 10 ways the movie could either rake in major box-office bucks or epically fail in a way not seen since Speed Racer dropped off the face of the planet. Even Pattinson might not be enough to save this one.
1. Bella and Edward's Wedding
The wedding and first 100 pages of Breaking Dawn is, without a doubt, the most entertaining part of the book. It’s like old style Twilight, with every character acting as they should (except for Edward and the garter ... weird much?), and the ever-present Bella-Edward-Jacob love triangle. Unfortunately, the most anticipated event since the Millennium is over all too quickly and things start to get worse as soon as our vampire and his bride reach their honeymoon destination, the Isle Esme. When Breaking Dawn is finally made into a movie, more emphasis should be put into the wedding: It’s the last time a lot of fans felt like they were reading The Twilight Saga and not some dodgy fan-fic found on the Internet.
2. The "Sex" Scenes
Yep, you read that right. Bella and Edward finally get it on, in what can only be described as the most absent sex scene written since the 1900s. The act itself is merely implied, with the only proof it happened some broken headboards and bitten pillows. Oh, and a very bruised Bella. How they’ll portray this in the movie is anyone’s guess, and no doubt complaints of abuse and domestic violence are a big fear of everyone involved with the movie. If it stays rated PG-13 like its predecessors, ardent fans of Bella and Edward can kiss goodbye to their much adored night of passion. Sorry kids, it’s still implication only.
4. Renesmee's Birth