Americans Hate Tingle: Longchamp Naito is near-universally despised by a large percentage of overseas fans, due largely to his goofy appearance and rambunctious personality. He doesn't get nearly as much hate by the Japanese fandom, and even has a small following who lament the fact that he was removed from the anime due to his unpopularity.
Angst? What Angst?: Lampshaded and subverted; in the Future Arc Dino notes that Yamamoto seems awfully chipper for having just suffered a great defeat (alongside being stuck in a future where his father's been murdered). It's partly a facade and Yamamoto later admits to Squalo in private that it's been eating at him.
Byakuran, despite all the build-up, especially being a Multiversal Conqueror of almost every universe in existence except the one Tsuna lives in and having all the knowledge and strength of those worlds, gets defeated swiftly in six chapters that reeks heavily of Deus ex Machina.
Nearly all fights from the Curse of the Rainbow feel anti-climatic. Whenever somebody is taking the upper hand, the fight is briefly stopped or simply cancelled.
Bermuda, despite being a Cursed Arcobaleno and one of the strongest villains in the entire series, goes down by Tsuna's new Super Mode in one chapter. Immediately after this, Checker Face reveals himself to be a Well-Intentioned Extremist and gives in to Tsuna's peace negotiations.
Most people watching or reading the series for the first time get tired of the Daily Life arc very quickly, since its mostly wacky, repetitive hijinks meant to introduce the main cast. This can be chalked up to Early Installment Weirdness; the series started out as a gag manga and didn't become action-oriented until later.
The Future arc which, at the time of its completion, made up over half of the manga. Was even longer in the anime due to two additional filler arcs in the middle.
The only way to explain what happened to Adelheid Suzuki: she went from fighting evenly with Hibari to not being able to land a single blow on him within slightly more than a week. This is especially egregious when you consider that not only did she get a massive power boost from the "Vongola Sin", it was expressly stated that this power would continue to grow over the course of the next seven days.
Tsuna goes through this in every single arc. He manages to defeat the Big Bad from each arc, but in the next one is curbstomped by the first enemy he faces and ends up feeling completely insecure about it.
Hibari: Half of the fandom loves him because of his Memetic Badass anti-hero status; the other half hates him, seeing him as a Creator's PetJerk Sue. It doesn't help that we see the other characters have to endure Training from Hell multiple times, something that Hibari himself never has to do, and every fight he participates in is anticlimactic.
Tsuna: Many see his reluctance to fight as a nice change from the cliche action hero. However, many other fans are annoyed that he's still a whiny idiot who keeps losing at the beginning of each arc.
Iemitsu, Tsuna's father, often comes under criticism for his negligence towards Tsuna and Nana (especially so in the final arc, where he nearly kills Tsuna during a poorly-communicated Stealth Mentor moment). Others feel that the fact he's often protecting his family while working for CEDEF justifies the neglect.
YamamotoxWall was a common joke during the early days of the fandom, and is still remembered rather fondly. explanation In his fight with Genkishi, Yamamoto has the upper hand right up until Genkishi tricks him into running headfirst into a reinforced steel wall, effectively OHKO'ing him. For some the scene was so jarring that it was hilarious, and thus the joke ship took off.
HibarixNamimori. That is, shipping Hibari with the actual school building.
Complete Monster: The future version of Byakuran from this Shonen Jump series is the Boss of the Gesso Famiglia and the Big Bad of the Future Arc. Using his ability to to communicate with and gain knowledge from every other Byakuran in a different parallel universe, he conquered each and every one of them until the only one left was the main universe. He planned to cement his grip on all reality by obtaining the Tri-ni-set; first ordering the slaughter of the Vongola family itself and their friends and families, then releasing Non Tri-ni-set radiation into the atmosphere in order to kill all Arcobaleno members. While he displays a cheerful disposition around his subordinates, it's only if they hold his interest. In reality, he's ruthlessness and cruel. Once he "gets bored" of someone, he won't hesitate to have them killed. He will do anything to achieve his goal, not caring if it would mean sacrificing innocent people (who he refers to as his toys) or destroying the world.
Draco in Leather Pants: Almost every single villain in the series has received this treatment; they can do anything from murdering children to enacting mass genocide by conquering the multiverse, and yet still be beloved by fans due to being attractive and/or charismatic.
Ear Worm: A good number of the anime's openings and endings, as well as some of the character songs can be guilty of becoming this.
Twin fics are VERY common in the fanfiction community. With the younger twin usually being a bully to Tsuna for no reason other than the fact that he's Book Dumb. In fact, twin fics where the younger twin truly cared about Tsuna regardless of that are so scarce.
Fem! Tsuna fics. No, really.
Like Naruto, there are nail fics where Tsuna got help from a random crossover character of choice/an OC. Making him a bit more better than canon by the time the story starts.
Fanfic Fuel: There are MANY fics in regards to what happened to Tsuna and his guardians after chapter 409. Most have it involve the fact that he had accepted the role of the Decimo or the troubles he had to face as the official Decimo.
There are also many class reunion fics in regards to said theme above with their old classmates finding the truth about Tsuna's heritage.
Mukuro/Tsuna shippers take Mukuro's will to "Take Tsuna's body" in a very different way than the one meant.
The same above is said to all ships including Hibari, but Hibari/Tsuna in particular, towards his catchphrase "I will bite you to death".
A generally accepted headcanon amongst the fandom is that Yamamoto's mother is deceased (given that she's never seen or mentioned despite Yamamoto's close relationship with his father). Likewise, it's often inferred that Hibari's parents are absent, or simply negligent.
Kaoru is the only member of the Simon family not to have revealed his Earth Flame. People have thus theorized it to be either Ocean or Meadow, given that each Simon member has had the kanji for their Flame name in their actual name.
A good number of fans in the fandom believed that Tsuna's hyper intuition and flame being sealed caused an imbalance which was the reason why Tsuna is so clumsy and Book Dumb.
Fashion-Victim Villain: Daemon Spade. For the most part the Evil Is Sexy trope is in effect... and then he went and possessed Mukuro. Now we have this. Standard fandom response is laughing your ass off.
Growing the Beard: By the Varia arc; the Kokuyo arc was the first to be Darker and Edgier, but the Varia arc did wonders in world-building and generally having a more engaging plot.
Harsher in Hindsight: Reborn's early interactions with Lambo. Sure, we know how annoying Lambo is about the "rivalry" thing and that he's not gonna be killed even by grenades, but once you realize it's an adult hitman beating the living crap out of 5-year old kid...
Tsuna. Shipped with his Bishōnen Guardians, almost every girl his age, his allies, his enemies, the list goes on. There's actually a name for shipping Tsuna and basically everybody else in the series: All 27 or All/Tsuna.
Chrome, by virtue of being the only female amongst the guardians.
Byakuran definitely fits the bill. In every single one of the multiverses.
With the reveal of who really masterminded the time travel plot, TYL!Tsuna gets props for putting the kids in significant danger just to motivate his younger self. His own younger self can't believe it was really him.
Depraved Homosexual Lussuria of the Varia. Who during his fight with Ryohei expresses blatant interest in him while simultaneously reveling himself to be a necrophiliac.
Glo Xinia. It is heavily implied during his encounter with Chrome that if Mukuro hadn't stepped in when he did, Glo probably would have tried to rape her.
Spanner's wildly regarded as a pedophile due to his interactions with Tsuna, which included stripping him for no real reason and handcuffing him to his bed. His blushing face (in regards to the X-Burner technically) and sudden Heel–Face Turn don't help.
Byakuran smiles constantly, loves "symbolic flowers", kills off one of KHR's other memetic rapists, and has signature moves where he 'penetrates you'. Oh and let's not forget his image song where he asks you to 'give him your body'. Then there's his blatant obsession with Yuni once she escapes from his grasp. Although there is a good reason for it, the vibes are still there. Not to mention his grabbing of Tsuna from behind and whispering into his ear...
Julie Katou wants you to know that you have "date plans" with him. And he will help you dress for them~ Though it is all due to Daemon Spade possessing him.
Daemon Spade's a pedophile that wants your body. Nufufu~How ironic that one of the Memetic Molesters above is a victim for this guy.
Moe: A lot of the girls can qualify, but especially wide-eyed Shrinking Violet Chrome. Tsuna is a male example.
5-year-old Lambo. Most of the fans find his Bratty Half-Pint tendencies annoying rather than endearing.
M.M. isn't very liked at all, due to her bratty personality and the horrible way she acts towards Chrome (though for some reason her treatment of Chrome is given much more hate than Ken's equally bad treatment of her).
Iemitsu. He's frequently given Ron the Death Eater treatment due to his raising of Tsuna — to be more specific, the lack of it, which fanon believes had a detrimental effect on Tsuna's emotional growth during childhood (which actually has some Truth in Television). As far as many fans are concerned, Tsuna's father is Reborn, since he did more to support Tsuna than Iemitsu ever did.
Shocking Swerve: The real identity of Checker Face is Kawahira, the Ramen shop owner...who is that again? Not only that, but he and Yuni's ancestor are secretly from an ancient immortal species that have existed on Earth since before humans!
Squick: Gamma admits to having an unrequited love towards his boss Aria, and when she passes away he says he feels the same way about her daughter Yuni (which Yuni appears to reciprocate). The problem is that Gamma is in his thirties while Yuni is fourteen or younger.
There were many who were Demoted to Extra in the final arc, such as Ryohei, Lambo, and even Yamamoto and Gokudera. Especially jarring since they had just received brand new Cambio Forma weaponry in the previous arc, which Gokudera and Yamamoto had barely shown off.
20YL!Lambo, the mature, handsome, and incredibly Bad Ass future version of 5-year-old Lambo, only ever appears in one chapter. It's justified that he can't show up in the Future Arc due to Lambo not being in possession of his 10-Year Bazooka, but even afterwards it's only 10YL!Lambo that makes a reappearance.
The Arcobaleno Arc could've been great, if it wasn't for its premature ending.
Mukuro and Hibari never got their rematch despite the build up. They do get to fight again in one of the anime's filler arcs, but it's cut short by Tsuna's intervention.
Uncanny Valley: Bermuda's true form resembles a doll whose proportions are too unbalanced. His eyes are too large for his baby like face, has a nose that is practically nonexistent and has the arms of a bodybuilder attached to an adolescent body.
Mammon's gender is often debated between fans. Due to being a baby wearing an eye-concealing hood their features and voice are both gender-neutral (as are the pronouns used to refer to them). Viz Media refers to them as male in english translations, and they're grouped into the male character category in the third popularity poll.
Kangaryu, Ryohei's box weapon, is female. Isn't helped at all by the fact that some people translate her name as Mangaryu.
Wangst: A strange example. Tsuna's life is supposed to have gotten better after meeting Reborn, but Byakuran tortures him by saying how much his life sucked during the Future Arc. Ironically, he is kind of right considering how at the beginning Tsuna was exasperated with the idea of not going home while his friends were trying to stay calm about it.
Gokudera, especially when you learn about his past.
Chrome, due to the hell she goes through. Her parents abandoned her to die, half her organs are missing, she's homeless and more often than not starving, she's constantly belittled, and in the Inheritance Ceremony arc she's hypnotized and tortured by D.Spade purely to push Mukuro's Berserk Button.
20YL!Lambo, particularly when you realize, thanks to Byakuran, exactly what kind of future he has to return to.
Yuni, and by extension Luche and Aria. Luche was Blessed with Suck, received the less-than-beneficial end of the Cursed with Awesome stick shared by her other partners, and worse yet has to pass that on to all her future descendents. Yuni has to sacrifice herself in the Future Arc to restore the Arcobaleno and in turn defeat Byakuran, but she's terrified of death and can't go through with it until Gamma joins her and they die together. The scene is regarded as one of the most tear-jerking of the manga/anime.