The Examiner's Twilight Examiner, Amanda Bell, has a great article discussing which scenes from the book are imperative for the film.
Bella's birthday - Ah, yes. A personal favorite of mine, and also a part of chapter one in Meyer's New Moon, the birthday scene is one for the books. It involves Bella unwillingly attending her own birthday party (a product of Alice Cullen's incessant, but lovable, party-planning-persona), getting a paper-cut and nearly being mauled by Jasper.
Break-up - After Jasper's near attack, Edward becomes estranged from Bella, and it is heart-breaking to her even then. I dare not describe here what the scene entails for fear of sullying it with my own unworthy descriptions. Let's just say that the conversation between Bella and Edward which leads him to depart, to the fans, needs to be spot on. Gut-wrenching as it was in the book, you say, so it must be in the movie. You also want to see the disappearance of all things Edward, as promised, from Bella's possession (another factor which rips Bella Swan apart in the story).
Read the entire article here
(WARNING: This will contain spoilers for anyone who hasn't read New Moon)