The Bad Guy Wins: Sarah gets her way in the book since Anna dies in a way that's perfect for her organs to be harvested for Kate, which was what she wanted from the start.
Broken Base: Whether the book or the film's ending is better is a massive point of contention. The book ends with Anna suddenly dying in a car accident, and Kate receiving her kidney. The film avoids this, so Kate dies and Anna lives. Fans of the book's ending typically appreciate its themes about fragility and the randomness of life, as well as the fact that Anna does end up saving Kate's life after all. This disappears in the film's ending. Detractors typically hate the book's ending for coming out of nowhere, being a Writer Cop Out to avoid answering the moral dilemma the book presents, rendering a big portion of the book, namely the entire main plotpointless, and letting the the selfish and abusiveSarahget her way, while the film's ending has none of the aforementioned problems. Chances are that whatever side a person is on, they'll think one of the endings is great and the other ruins the story.
Diabolus ex Machina / Shocking Swerve: After winning the case, Anna is killed in a car accident and ends being brain dead. Her kidney is donated to Kate, rendering the whole thing pointless.
Averted in the film. Kate dies with dignity and Anna lives.
Fridge Brilliance: We assume that Anna is the narrator in the prologue and it is Anna who tried to kill her sister Kate when she was three years old. It is actually Kate. The only other time she narrates is at the end. Note the use of the italics.
Sara in the film is shown to be far more sympathetic, some of her abuse being toned down and ultimately getting a Heel–Face Turn having to come to terms with Kate's death.
Likewise to those who view Anna as a Sympathetic Sue in the book, she's less like this in the film. With some of Sara's abuse toned down and without her Deus Angst Machina death, some of the Sueishness is downplayed. And since the film shows things from outside Anna's perspective, that helps too.
Romantic Plot Tumor: Campbell and Julia's relationship takes up a lot of pages for something that isn't really related to the more compelling main plot and could be removed or edited down without any real effect on the story, at times taking up the brunt of a chapter to explain their history when Anna's situation always feels far more critical. Notably, the film streamlines the plot by completely removing Julia's character.
The Scrappy: Sara, for being an emotionally abusive mother towards Anna.
Tear Jerker: In the movie, Sara breaking down in front of Kate and Anna's narration telling us that Kate died that night.
Jesse's motivations. He was almost completely ignored after Kate was diagnosed and Anna was born because he wasn't a match for Kate, and ended up destroying things because he'd never been able to fix them.
Wangst: Sara. A lot. All she ever talked about from when Kate was diagnosed to when Anna sued them was how sick Kate was. Lampshaded when her sister Zanne says, "You can't be a martyr all the time," and Sara mistakes her for saying "You can't be a mother all the time."
It should be clarified that Sara's martyr complex toward Kate's illness borders into some disturbingly narcissistic areas at some points, often using her "maternal instinct" and suffering to justify behavior that is outright neglectful, if not emotionally manipulative. Of example -
When Brian, the father, gives Anna the locket she pawns earlier in the book, Sara justifies shock at seeing him reward Anna's gift of marrow as (paraphrasing) "not occurring to her that suffering was worth rewarding, since they'd all been through so much of it." This is after she nearly refuses to see Anna after her operation (a very painful one, at that), claiming that she's too busy with Kate.
Outright admitting that she's given up on Jesse in the narrative, dismissing his emotional claim that she "doesn't know what it's like to be the kid whose sister is dying of cancer" by saying her experience as the mother of said child trumps his. She also accuses him of being a drug addict, only to be shocked into silence when he angrily explains that the injection marks are from donating platelets for Kate.
She repeatedly lies or dismisses Anna's decision to sue to both Campbell and various characters, saying that it's all a misunderstanding.
In one scene after interviewing with Campbell over the lawsuit, Sara accuses outright that Anna has "signed her sister's death warrant" because of it.
Even after the court hearing, where the revelation of Kate's support of Anna's lawsuit, Sara insists that while what she did may not have been fair, moral, or ethical, it was surely "right." In fact, the only apology she ever issues in the book is to Kate, not Anna or Jesse.
Sara's complex penetrates deep enough psychologically that it impacts even Kate. In a very telling scene in the novel, Kate, frustrated and overwhelmed by the constant regime of recurrent hospitalization, declares lucidly that she's had enough of it all. Instead of approaching her to speak honestly about it, Sara states with startling vituperation that it's "[Kate's] suicide."
The Woobie: Kate herself, through and through. Also, her sister, Anna. Even Jesse has moments of sympathy despite his actions due to the treatment from his mother all because he's not a match for Kate.
Jerkass Woobie: In the movie adaptation, even the wangsting Sara gets her sympathetic moments when she broke down by Kate's side as her dying daughter in her deathbed slowly passes away in the same night.