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.
Adaptational Attractiveness: In the book, Eric is described as having a frightening and repugnant appearance. In the film, he certainly still has an intimidating presence, but is actually pretty good looking.
Tris herself - possibly. Opinion varies on whether, in the book, she's being accurate when she describes herself as 'nothing special' verging on 'plain'. Shailene Woodley is firmly on the attractive side of the scale.
The books have a minor Running Gag of Tris being mocked for having an overly large nose, which Woodley does not suffer from by any means.
Is Tris a brave, compassionate, sharp but self-doubting heroine who is worth rooting for, or is she an angsty, self-absorbed person with little human decency despite her upbringing, and some sort of constant mild concussion that blinds her to the bleedin' obvious?
Was Eric's eulogy to Al a sincere Pet the Dog moment, or was it a lie simply to make himself look good? While Tris thinks it's the latter, Living Lie Detector Christina didn't point it out.
Broken Base: The announcement that Uriah will be cut from the first movie and saved for the sequel. Some believe that Uriah's purpose in the first book wasn't that important, while others that love the character still call heresy on it. It's Tom Bombadil all over again.
The ending of Allegiant has severely divided the fanbase, with the reviews on Amazon averaging 2.5 stars when it first came out, but has gone up to 3.5.
Crowning Music of Awesome: Woodkid's 'Run Boy Run', an instrumental version of which essentially becomes the Dauntless theme music in the first movie.
Expy: Humans trying to perfect humanity through genetic engineering, the new humans being incredibly cruel and vicious, and a massive war starting over these genetically modified humans in which populations are decimated? It's practically what the Eugenics Wars would have been like if the Augments didn't get political power.
Fridge Logic: The worldbuilding is pretty vague and the social caste system in the society is rather simplistic and one-dimensional. Might be Fridge Brilliance in that the whole point of the story is to tear down such a system and demonstrate why it would not work.
Also makes a bit more sense when Allegiant reveals that their entire system was set up by a government group, which is still actively working to keep the faction system going through subtle interference and the occasional mass memory wipe.
Hollywood Geography: Dauntless territory apparently stretches from the Hancock building to Navy Pier, among other inconsistencies.
Hype Backlash: Fan reaction to Insurgent and particularly Allegiant has been far milder and in some cases even downright negative in comparison to how Divergent was received.
Numerous candidates for when Peter crossed this. Shining examples are how he stabbed Edward in the eye while he was sleeping. Then there was the time he hung Tris over the chasm (claimed it was just to scare her) and groped her chest, while mocking her for "feeling twelve".
For Caleb, betraying his family to the Erudite. The full information about the outside world and what Jeanine told him has yet to be disclosed. But regardless, it doesn't change the fact that he sided with the faction that killed his parents and helped with the capture and attempted execution of his own sister.
Narm: Some reviewers have pointed out that while Dauntless's habit of doing Le Parkour to board the overhead railway is clearly meant to embody how free spirited and brave they are the movie makes them look more like the cast of Fame or West Side Story than army recruits. Mark Kermode in his BBC review also thought their jumpsuits looked "a bit Blake's 7", which may become Narm Charm for some viewers.
Moreso in Allegiant when he reveals that he knows he's a horrible person and disgusts himself but can't help it and decides to wipe his own memory in hopes of becoming better. Makes him almost cross into Jerk Ass Woobie territory.
Take That, Scrappy!: If Peter, Molly, and Drew do something to irritate you, just breath a sigh of relief that they'll be humiliated for it later by Tris' hands.
Marcus gets one in Insurgent when Tobias gets laughed at for fearing him. So, to prove that he's no coward, he gives Marcus a well-deserved beat-down.
The Woobie: Four. He's actually an Abnegation transfer named Tobias who left because his father, Marcus, constantly abused him.
Al is arguably the biggest one in the entire series. He didn't belong in Dauntless from the get-go and only joined to make his family proud, when he clearly belonged in somewhere like Amity. His entire initiation experience is agonizing and terrifying to him, and he grows more and more distant until eventually, when it's clear that his only choices are factionlessness and death, he joins with Peter and Drew in their attempt to murder Tris, regretting it immediately afterwards. It gets worse when Tris coldly rejects his pleas for redemption, despite being fully aware of his mental state and the intentions behind his actions, which leads to his suicide. A major What the Hell, Hero? moment for many readers.
Tris, by the end of the first book. During initiation, she is bullied, verbally harassed, and sexually assaulted for being a "Stiff", becomes a victim of attempted murder, drives one of her friends to suicide, watches her entire world and society crumble around her, is forced to shoot one of her best friends (who was mind-controlled), loses both of her parents, and is nearly shot and killed by the boy she loves (who was also mind-controlled). Her mental state continues to spiral downwards in Insurgent, to the point where she is nearly Driven to Suicide in the Candor Headquarters, and only holds back because she thought it would be ungrateful towards her parents, who sacrificed their lives for her. She also turns herself in to Jeanine without any escape plan, and the text makes it abundantly clear that she actively wants to die at this point.
Trauma Conga Line: From the ending of the first book onwards, almost every character crosses this.
Unfortunate Implications: Smart people are inherently evil, and if anyone in your family is more intelligent than average they will betray you to other intelligent people.
Fortunately, the latter two books are much better about this than the first. The introduction of Cara, along with the growing implications that both Tris and her father were Unreliable Narrators in regards to the Erudite, do wonders.
Many a fan's outrage at Tris' rejection of Al's apology is this for a few readers, as it seems to carry the implication that victims of assault and abuse need to forgive their abusers in order to be a good person.
Wangst: Depending on the reader, Tris's endless agonization over everything might be off-putting.