These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
This one is more prevalent in the film rather than the book. Is Amy just pure evil or a Jerkass Woobie? Given she was willing to kill herself as the major part of her plan and wants to come home after Nick shows he's capable of being the husband she remembers, it seems possible to view her actions as not simple psychosis but being desperately unhappy. Rosamund Pike's performance also seems to indicate that Amy genuinely loved Nick in her own way but felt betrayed by his attempt to box her in as his unwanted nagging wife from New York. This is despite the fact she moved across the country to a small town in order to be with him.
Oddly, averted with Nick. Is Nick the grieving husband, a Nice Guy, and cool sophisticate who is the first person to ever treat Amy as a person? Or is Nick a boderline misogynist, a controlling asshole, and unambitious loser who gave up on a prestigious life in New York to become a community college professor who sleeps with much-younger students? The answer is, he's all of these things. Nick is a multifaceted guy with both dark and light qualities; qualities which come to the fore depending on his circumstances or desire to show them.
Nick managing to turn the tide just a little bit and get some of the public back on his side with the video of him, half-drunk, professing his love for Amy and begging her to come back. It works so well that even Amy herself was convinced to return.
Near the end of the book Nick torpedo's Amy's latest scheme to keep him in line (framing him for poisoning her) showing that now that he knows how the game is played he can play it as good as Amy does.
Broken Base: The Cool Girl speech has divided feminists over whether Amy is making a good point about women not changing themselves to fit the ideals of men or whether she's just condemning women who don't fit her ideal of what is proper behavior. The movie subtly argues that Amy is wrong by singling out women, failing to see that humans in general behave that way - this is highlighted by her wrongly identifying one of the car passengers she passes by, who is sitting with another woman, as a Cool Girl.
Critical Research Failure : A mild but still annoying example in the book. When Nick enters the wrong code for his dad's house and trips the alarm, he calls the monitoring station. They turn it off for him, which can't be done remotely.
Nick: And then all we did was resent each other, and try to control each other. And cause each other pain.
Amy: That's marriage.
Jerkass Woobie: Amy does terrible things to people who were repentant about their mistakes, but it's implied that being brought up to be 'Amazing Amy' left some considerable emotional damage on her, "I remember always being baffled by other children. I would be at a birthday party and watch the other kids giggling and making faces, and I would try to do that too, but I wouldn't understand why." Her parents expected her to be perfect, and each day was an opportunity to fail.
Magnificent Bitch: Amy's Evil Plan is detailed, extensive, and required a lot of research to pull off. Her pre-game is so effective all she has to do is sit back and watch the world consume her adulterous husband.
"I'm the cunt you married. The only time you liked yourself was when you were trying to be someone this cunt might like. I'm not a quitter, I'm that cunt. I killed for you; who else can say that? You think you'd be happy with a nice Midwestern girl? No way, baby! I'm it." Variations of this speech are starting to pop up on Tumblr and other sites.
Misaimed Fandom: Some fans legitimately believe that Amy is a feminist role model despite (or because of?) her being a complete and utter manipulative sociopath, up to applauding her murder of Desi and her framing of Nick.
The Woobie: Go is probably the biggest contender- she has to helplessly watch as her twin brother is accused of murdering his own wife, discovering that he has lied to her and has the press constantly asking if she was involved in Amy's disappearance. By the end of the story she also has to live with the knowledge that her brother intends to stay with a manipulative, murderous psychopath.
Esoteric Happy Ending: The ending is deliberately written to be this. Amy finally has someone who she can be herself with and Nick has the child he always wanted. They may eventually kill each other but, at heart, they're both sociopaths.
Genius Bonus: When Nick is recounting Amy's obsession with finishing everything, he remembers her reading Haruki Murakami's "The Wind Up Bird Chronicle", which is about an estranged couple, along with the wife suddenly going missing.
Shocking Swerve: A book-exclusive one: Amy shows up bloodied on Nick's doorstep after murdering Desi. The movie averts this by showing the earlier of the two events in question before showing the one that takes place chronologically later.
Nick comes across as less of a sociopath in the film; he's prone to emotion, genuinely cares about people, and empathizes. To be sure, he's a schlubby kind of guy and painfully awkward when out of his comfort zone (read: the entire movie), but his faults are presented as more of the garden-variety mid-life crisis type. This more human characterisation has the side effect of making the already dark ending bleaker in the film than in the book. Book Nick seems excited by Amy and willing and able to manipulate her in turn; film Nick comes across as disgusted but powerless and any fascination with Amy is more akin to Stockholm Syndrome than anything else.
That may not be entirely true. In a blink and you'll miss it moment, Nick's throwaway line of them being "partners in crime" seems to imply he's not quite the trapped victim Amy hoped from, especially after their earlier confrontation. Amy looks visibly surprised by the comment.
Andie comes off as more shallow and vapid than her book counterpart thanks to a comparative lack of screentime- was she sleeping with Nick because she genuinely loved him, or did she just think that it might give her a leg up in her career?
You can be much happier in marriage if you both pretend to be better people than you are.
Don't cheat on your spouse, ever.
Never get married.
Don't get married to a sociopath.
Author's Saving Throw: The movie shows a scene that was only discussed in the book, but was criticized for not including the event in the narrative: Amy's murder of Desi, in all of its gory details.
Award Snub: Was only nominated for Best Actress at the Oscars. No nominations for Gillian Flynn for Adapted Screenplay, David Fincher for Director, or Trent Reznor/Atticus Ross for Original Score, or Best Picture.
Awesome Music: As always, Trent Reznor and Atticus Ross' score is a highlight. Especially the tracks that plays over Nick discovering the contents of the woodshed, the "Cool Girl" speech and Amy's murder of Desi.
Broken Aesop: Amy's Cool Girl speech about being true to yourself is undermined by the fact that she seems as honest with Nick in her flashbacks as she's ever with anyone. Therefore, her anger seems misplaced. Likewise, Amy is incredibly unhappy when she's left to be herself and quickly falls into old patterns in order to make friends. Not to mention that one woman in a car that she focuses on as a "Cool Girl" turns out to be sitting next to another woman, not a man. It undermines Amy's point that women only act in certain ways to draw male attention, as opposed to simply being the way humans act for each other. Of course, given that Amy is a sociopath with narcissistic traits, the Aesop-breaking is likely deliberate.