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.
Broken Base: Everyone generally likes the novel, and will typically also like either the film or the manga, but rarely both. Any of the three may be preferred though, and huge arguments are inevitable when trying to posit one version over the other. Especially on the most active forum for it, which is at IMDB. A film forum.
Yonemi Kamon (manga) is the administrator of the Battle Royale Program, where high school students are forced to kill one another. His hobbies are watching students kill each other, killing students and joking about said students' deaths. After taking a school bus full of students including Shuuya and Yoshio to be transported into the Program, he later mentions that he also went and raped an orphanage director Shuuya and Yoshio had grown close to. Unlike the movie portrayal of the Director, Yonemi doesn't even pretend to buy into the crap about maintaining order or protecting the honor of the Empire. He enjoys raping and killing, and is a huge fan of the Program because he gets to see teenagers deal with gut-wrenching emotions before dying horribly. Even soldiers are visibly nervous around him.
Kazuo Kiriyama from The Film of the Book stands in contrast to his emotionless counterpart from the book and manga versions. Kiriyama in the film is a ruthless and psychopathic Blood Knight who voluntarily signed up for 'The Program' to be able to hunt down teenagersfor fun. Throughout the film, Kiriyama wracks up the highest kill count, not caring if his victims are helpless or not, and after gunning down one girl, he uses a megaphone so everyone around can hear her pain and his execution of her. While he never speaks a word, his Slasher Smile throughout the film speaks volumes for how much he is relishing his murder of everyone around him.
Contested Sequel: The film sequel is... not popular. There's a reason it hasn't gotten any adaptations in any other media.
The most memorable example is Toshinori's death; in the original novel, Kazuo merely shoots him in the face, but in the movie, Kazuo chops his head off with a sword and then stuffs a grenade into his mouth, using the head as a bomb.
Kyoichi tries to shoot Shuya, but Shogo blows off his arm. Now most of us would have give up there, but Kyoichi goes for his fallen arm, pries the gun loose with his teeth, and tries again. Only to take a shotgun blast through the gut. At that point, he would probably have tried to throw his entrails at Shogo, but the author decided to let him rest.
Another occurs in the film version during Mitsuko's death. Kiriyama shoots her point blank in the chest: she gets back up. He shoots her again: she gets back up. He shoots her again: she gets back up. She finally dies after the fourth shot.
Kitano's death scene in the film. Having just revealed that he thought of Noriko as a daughter, he wants her to kill him, even threatening to shoot Noriko if she doesn't fire first. Instead, Shuya whips out his gun in retaliation, shooting Kitano to protect Noriko; as he falls, Kitano fires, showing that the gun in his hand was a mere water gun. A still-jittery Shuya fires again. Then, Kitano's cell phone starts ringing. Kitano, seen lying on the floor, presumably dead for a good seven or so seconds, immediately rises to his feet like nothing happened and answers the phone. He soon gets into an argument with his daughter Shiori, and in anger, throws the phone across the room and pulls out a real gun to blow it to smithereens. And if that wasn't enough, he samples the last of Noriko's cookies and remarks "Damn good cookies" before finally dying for good on a couch. It is impossible to take this part seriously.
Draco in Leather Pants: The prevalent fandom attitude towards Mitsuko, who is a backstabber, a murderer (as well as the only student to have killed someone before the Program), and possibly even a rapist - explicitly so in the manga. May have been designed as such, the author having admitted she's his favourite female character.
Despite the film's sequel not being well received, several students fit this trope amongst fans of the franchise. From the boys, they are Osamu Kasai and Masami Shibaki, due to their role in the final battle, and amongst the girls, they are Yuko Natsukawa, Rena Niima, Ryoko Hata, Kazumi Fukuda and Hibiki Yano, due to their more stand-out looks when compared to the rest of the female students.
Fanon Discontinuity: Almost (though not quite) universally, the second film. Due to the Broken Base however, all three versions have been hit with this depending on which version is favoured by that particular fan.
Fanon: Mitsuko is either a Depraved Bisexual or Psycho Lesbian. There is only one implication in the novel as evidence, as Mitsuko obviously prefers men to woman. It's batted about as commonly as "Shuya is the hero" (which is true) however, and fanfiction featuring Mitsuko that don't put her as one of those is all but non-existent. Seemingly even her expies in Original Battle Royales require it.
I Am Not Shazam: The words "Battle Royale" are never uttered once in the original book; the "Alternate Universe" prologue makes it clear that the title is a reference to the kind of conflict the Program involves. In the manga, there's only one easy-to-miss All In The Manual reference to the "Greater Eastasia Combat Experiment 68, also known as the Battle Royale Act".
This does not apply to the other film version - there, the colloquial name for the Millenial Education Reform Act is "The Battle Royale Act". The logo also appears to be officially used by the government.
Idiot Plot: The beginning of the movie says that Japan is affected by high unemployment and a violent and rebellious youth. The senate decides solving that problem wasting government resources (which could be used for employment programs) on a program that makes kids hating adults and rewards the most violent one.
The program probably doesn't work very well because none of the kids knows that the program exists. Maybe only kids who are Too Dumb to Live are attending school.
Making matters worse, it sows dissent and hatred at the government among the parents of the children who are kidnapped and brutally slaughtered for no damn reason whatsoever. It also isn't targeted at kids who show any kind of deviant or violent tendencies, but rather puts everyone in a violent, unforgiving, chaotic, and merciless death game where the most violent and deviant kids are at the greatest advantage. The issue isn't the premise of a death game among a selected class of students, but rather the total absence of a setting that makes any sense as to why something like this could possibly exist. The Hunger Games, by contrast, provides a plausible setting for a death game; Battle Royale is pretty much 100% Excuse Plot with an extra heaping of lazy.
Internet Backdraft: Expressing an opinion on forums of any kind regarding the proposed, stuck in Development Hell American remake should only be done if you enjoy being ritually disembowelled. Opposing it will get you accusations of being an It's Popular, Now It Sucks guy who fears it becoming mainstream, supporting it makes you a perpetuator of the rubbish Hollywood churns out via conveyor belt these days.
Mitsuru. Yes, thug he may be, but it's hard to not feel at least a little bad for him (especially in the manga) when he spends the majority of his narrative time going on about how amazing Kazuo is and how he's sure they're going to win the Program, and how he's Kazuo's best friend... only to be shot by him and realize in his dying moments that Kazuo never cared about him, nor about the rest of the gang, at all.
Mary Sue: In the novel and manga: Shuya, Mitsuko, Shinji and Kazuo. Yes, half of the main cast. Be wary of claiming any of the above is/isn't a Sue in any of the three media unless you have either a death wish or an extremely long essay by way of justification. Consensus among the fandom seems to be that the film largely removed the Sues, though Mitsuko remains a borderline and the main reason for Kazuo not being one is that he doesn't have enough personality to be considered a character at all.
Arguably, Noriko becomes one in the manga, but is far more well rounded and developed in the film and novel.
Misaimed Fandom: While the story is about the terror of the violent, hopeless situation, a lot of young viewers see it as a cool action flick.
Some quasi-sociopath fans seemingly wish they could enter the program with their class.
Amazingly, there are fansites dedicated to admiring Kazuo.
Moral Event Horizon: By default, you are forced to cross it if you want to survive the Program.
The politicians who set up the Program crossed it by default.
If Sakamochi didn't cross it by forcing a junior high school class to slaughter each other and happily telling them the rules, he definitely did when he raped the orphanage caretaker and then cruelly taunted Yoshitoki about it, causing him to explode with rage and get himself killed.
Kazuo Kiriyama has the most notable crossing, however, because he did it in such a gratuitously cruel and sadistic way; by gunning down Yumiko and Yukiko while they were sending a message by megaphone to stop fighting. The film version of Kiriyama takes this even further by turning on the megaphone and broadcasting Yumiko's dying cries, then shooting her several more times for good measure.
Kazushi Niida crosses this with his attack on Chigusa in all three versions.
Yuko Sakaki feels she has crossed the Moral Event Horizon when she poisons one of her friends, which causes suspicion and sets off the whole massacre in the lighthouse. When she understands what her action has caused, she is Driven to Suicide.
In the manga, Mitsuko raping a dying Yuichiro, although she wasn't exactly compos mentis at the time.
Nausea Fuel: Mitsuko's complete mental breakdown and slow death at Kazuo Kiriyama's hands in the manga. Also a bit of a Tear Jerker.
Nightmare Fuel: Almost every scene towards the end. The premise itself counts.
One-Scene Wonder: Almost anyone except the core half dozen may count depending on your preferences. Two are universally agreed upon though, one is Chigusa, who is definitely one of the best known characters despite having only two significant scenes, and they're consecutive. The other is Yukie Utsumi for the Lighthouse scene.
Paranoia Fuel: The premise itself is a wonderful/horrifying example of this trope. You'll never look at your friends the same way again after seeing/reading this.
Purity Sue: Noriko is considered by some people to be one of these.
Sequelitis: Battle Royale 2: Requiem, up to and including the hamfisted attempt to tie in the 9/11 attacks. The extended version, Battle Royale 2: Revenge is considered better than Requiem, though not to redeem the worst flaws. May be because there is only one novel, that doesn't go beyond the lore of the first movie.
Squick: The manga does not believe in the Discretion Shot. The novel and film aren't immune from this either, with all three versions of Niida's death being very brutal, and in the film Kazuo finishing off Yumiko by putting the megaphone to her mouth so that her dying moans are audible before firing his Uzi down the length of her torso definitely qualifies.
The manga also does not shy away nudity and rape scenes.
Thirty Sue Pileup: The entire main cast, in the novel. Arguably not changed in the manga, though the film does remove all but Mitsuko as a Mary Sue.
Too Cool to Live: Shinji Mimura, who, in opinion of many, at least in the manga does better job as a protagonist (in his subplot than the actual main character. Arguably near the end of the story Kawada also qualifies, because if he lived, he'd be overshadowing Shuuya in his victory over The Program.
Too Dumb to Live: Some of the students certainly seem that way - Yumiko and Yukiko who make themselves sitting ducks out in the open, actually broadcasting their location and simply hoping that they won't get killed. Shuuya himself as well, with his almost beyond-idiotic belief in his classmates, despite the many, many moments of evidence that show him the exact opposite.
Values Dissonance: Several instances where Japanese cultural ideas that would will be lost on unknowing Western audiences, such as Kitano nonsensically answering a phone call and eating the last cookie from the bag after he's been machine gunned and supposedly dropped dead on the couch. The classroom scene also qualifies as it parodies what is a very Japanese style of schooling.
Mitsuko in all three versions, though a loose translation of her final words in the film means that it's less clear in that. The book and manga go to great lengths to justify her actions on the basis of her Freudian Excuse (see above). The character wants the audience to feel sorry for her, and while her classmates don't fall for it, Takami seemingly does.