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.
Anti-Climax Boss: Once Shotaro and Philip got their act together, Terror was surprisingly easy to beat in a straight fight, most likely due to the point of the Gaia Memory being to terrify your opponents into a state of helplessness.
Make that Double/Accel worked together. Once Ryu drew the Terror Dragon off and beat it, Ryubee's combat effectiveness was drastically reduced (also notice the fact that Ryubee can only create those liquids when Terror Dragon stayed in his headdress).
Utopia as well. Despite defeating Accel and the Taboo Dopant with no effort, Double makes surprisingly short work against him. Justified in that his strongest ability, absorbing emotions to power himself up, was used to overload him.
He gets defeated again in a spin-off movie, though this time it's because his opponent is Dead to Begin With and thus has no emotions to draw power from.
Awesome Music: Wailing Desperation, the then nameless One-Woman Wail from the prologue of the first episode; it was really short, but having been presented at a crucial and dramatic moment, it left quite an impact.
You can hear it again at the end of the first part of "The Girl...A", but with a more subdued and chilling arrangement as it accompanies an important Reveal.
Double's three Battle Themes, each with their own fitting style.
"Glorious Street", the Heaven's Tornado song from episodes 7 and 8.
Accel's Battle Theme, "Leave All Behind", based on his catchphrase.
"Nobody's Perfect", Kamen Rider Skull's theme; sung by the Chief himself, Koji Kikkawa, at the climax of "The B Carried on the Wind".
And following that, an upbeating sound of a music as Double starts fighting against the Beast Dopant in his new Super Mode.
That same music plays again in Accel Trial's debut.
"Extreme Dream" which wasn't used on the show until ten episodes after it was released.
"Extreme Dream" is a very good example of how to properly use this trope. It was only played twice over the course of the series, both times during particularly climatic battles, rather then being spammed every fight scene after Double get's his Super Mode.
Same can be applied to "Leave All Behind", which was also used a total of two times.
Base Breaker: The fandom's general opinion of Akiko earlier in the series. Very few people in the fandom took the middle ground, but both sides mellowed out over time.
Big Lipped Alligator Moment: Shotaro fools Jinno into running away from an imaginary yeti at the end of "J's Labyrinth". It turns out there really is one, and Shotaro and Akiko faint.
While this is more of an event in the Kamen Rider Decade movie ''All Riders vs. Dai-Shocker," no one should forget Double's debut himself where he plastered Shadow Moon himself when the game-breaking Decade and Rising Ultimate Kuuga cannot.
Broken Base: Arguably, the series ending. Some fans preferred the Bittersweet Ending of #48 to serve as the series ending, whereas other fans liked the real ending (#49) as it was.
Doctor Shinkuro Isaka, the Weather Dopant. He killed Terui's family, modifies Gaia Memories to kill their users so that he can take their power for himself, and is able to use Gaia Memories via direct insertion without any ill effects (typically, repeated use of Gaia Memories via direct insertion warps the user's mind, which shows that Isaka didn't have far to fall). He then tries to get Saeko to kill her own father and take control of the Museum, but winds up getting Hoist by His Own Petard when all of the Gaia Memories he absorbed essentially destroy him from within upon Memory Break.
From W Returns: Eternal, we have Dr. Prospect aka the Eyes Dopant. This man took people from all over the world (their words implying they were kidnapped) to make them into, in his own words, "lab rats" for his Quark Project, giving them psychic powers. The fact that, when he arrives on the scene, every one these people with awesome Psychic Powers is quaking in fear of him shows how horrifically he must have treated them. That is nothing compared to his Heaven's Fall plan. This is a plan so horrible that the previously mentioned Utopia Dopant considered it a brutal act!He would blast the area with an electric pulse, killing anyone that has not lived up to his standards for the project, referring to it as "cleaning out the test pool". When Daido ruins that plan, he reveals that was merciful compared to what happens if they try to escape. The eyes he implanted on them to watch them kill them when they leave his compound. As Katsumi is in shock over the slaughter of the village he'd tried to save, Dr. Prospect is laughing his head off at the fact he'd just slaughtered a village of innocent people. His actions are so horrific that it drives Katsumi insane, a fact that serves as a valid Freudian Excuse for Katsumi's actions in the A-Z movie.
Draco in Leather Pants: An In-Universe example. A whole V-Cinema is devoted to a girl from Kamen Rider Eternal's past popping out of nowhere and explaining to Shotaro and Phillip that Daido wasn't a bad guy and was just a Fallen Hero. Some people don't buy the tragic backstory.
It's mitigated by the fact that Shotaro and Philip still point out that Eternal did a lot of bad stuff and hurt Fuuto badly. Their memorial to him comes off as bittersweet, thinking about what could have been for a man who became a monster but could have been a hero, given different circumstances.
Specific examples from within the series include Kamen Rider Joker, Sokichi Narumi/Kamen Rider Skull, and Kirihiko.
Skull especially. His Figuarts is notorious for managing to find popularity with people who weren't fans of the franchise and fans who weren't collectors.
Epileptic Trees: No, for once, it's not the "Female Riders, No Survivors" one. A single picture of the Core Medals set off a wildfire, convincing people that Foundation X would be the next Shocker. Seriously. While it didn't get an expected pay off, one aspect of the foreshadowing, the "King Medal" does make it into the finale of OOO...
The much later appearance of Foundation X in Kamen Rider Fourze would eventually confirm that Double, OOO and Fourze all do in fact take place within the same canon.
Although Akiko did henshin into Double with no ill effects...unless you count getting her face being sucked by the Utopia Dopant as the ill effect.
She only henshined in a dream and it was to combat the Nightmare Dopant. Utopia Dopant came 17 episodes later and sucked the faces off of pretty much everyone in Fuuto.
Hilarious in Hindsight: While no crossover with Samurai Sentai Shinkenger has been planned (unlike Kamen Rider Decade prior), two unintended parallels have surfaced - Memory Gadgets attach to Double's weapons while each carry one of the season's collectible items (the Gaia Memories much like the Shinkenmaru and Inroumaru with their Hiden Discs), and both Kirihiko and Mako's father married into their respective families.
Three now. In Shinkenger, it turned out that the real head of the Shiba family is a girl - fan reaction is mostly positive, but you let Akiko take charge of ONE detective agency, and out come the pitchforks...
In Kamen Rider Blade, Hirose attempts to crack the BOARD database and eventually figures out the password: "Double Joker".
Blade's Big Bad is, in fact, the Joker Undead. Not to mention that as Kamen Rider Chalice, he uses wind powers. Guess what Double's main form is called.
Ho Yay: Shotaro Hidari & Philip. The fact they share a body when in Extreme (or when just being Kamen Rider W) might have helped, but these two have generated a fair amount of theories based on their behavior.
Jerkass Woobie: Did bad stuff happen to the Puppeteer Dopant? Yes. Should you feel pity for the poor guy? Yes. Does that excuse any of the shit he pulls? No.
Mood Whiplash: Made the previous reigning king of this in the franchise, Kamen Rider Kabuto, look like an amateur effort. One example is how there would be serious scenes of Shotaro brooding over his mentor's death sandwiched between scenes of Akiko running around acting like a Looney Tunes character high on crystal meth.
Moral Event Horizon: If the powers of the Utopia Dopant don't convince you that Jun is a complete monster, then the fact that he went after Shotaro and Philip's friends and sucked their faces off will convince you to want to see Jun get his ass handed to.
Commander kidnapping Akiko with the intention to kill her in front of Terui to make Terui the copy of him. The fact she's an innocent person who has done nothing wrong is what makes him cross the line.
Narm: In The B Carried On The Wind, when Shotaro and Philip were walking towards the Beast Dopant near the end, Philip falls down, as usual when they transformed. What makes it hilarious is that they tranform in a semi-slowmo fashion, with the music "Nobody's Perfect", sung by Koji Kikkawa, the actor who plays Sokichi Narumi, their boss, and Philip literally falls on his face...In slowmo.
Even before that part, we got Terui being handed his behind on a platter. Slo-mo along the song. Pure gold.
Nausea Fuel: Deliberately invoked with "Find the C"'s Cockroach Dopant, whose apparently decorative appendages sometimes flutter involuntarily like actual cockroaches.
Nightmare Fuel: The Terror Dopant has the ability to make reality bleed the stuff.
Daido Katsumi going Laughing Mad when all the psychics are killed...
The way Isaka dies after Accel finally beats him in Trial form. Seriously, don't play the memory game, kids.
The Utopia Dopant lights Kamen Rider Accel on fire in Episode 47. As if that wasn't scary enough, Ryu Terui screams in horror as he is burning to a crisp both in and out ofhis rider form.
The Spider Dopant has the ability to implant spider bombs in people that make the people they love explode when they touch each other.
The whole Virus Dopant storyline. Incredibly depressing considering how Sachi was treated by the man she loved and ungodly scary if you see that the dopant isn't her physical body, but a manifestation of her hatred towards him and the guys who ran her.
Also, the way the dopant sounds and moves. It looks like something out of Resident Evil
Nightmare Retardant: The possessed doll from "The P's Game" would be absolutely terrifying, if it didn't make squeeky noises whenever it moved.
Special Effect Failure: Isaka's defeat. You'd think that by now they've stopped getting sloppy with the enemies exploding.
The movie has an example with all of Katsumi Daido's transformations. It's painfully obvious that he magically grows a foot whenever he becomes Eternal.
This Is Your Premise on Drugs: The Delusion Diaries, little shorts of Shotaro having really bizarre daydreams. Entries include 'what if Akiko was a little sister' (featuring the cast as his family, complete with Ryu as a housewife and the Weather Dopant as his father), 'what if Akiko was an idol' (ending in Akiko marrying the Triceratops Dopant) and 'what will the agency be like in the future' (featuring old man Shotaro, old woman Akiko, Ryu as a ghost haunting the agency, and Philip having not aged a day due to being data.)
Some western fans deride Kirihiko as a Henpecked Husband because he is deferential to Saeko, took her name after their marriage and has been physically abused by her on at least one occasion, along with the implication that it wasn't the first time. A Japanese audience would be less inclined to see him as weak, since their society emphasises reverence towards social superiors and loyalty to one's employer.
Hell, you don't have to be an expert on Japanese culture to see it working this way. Saeko is the Big Bad's daughter, and Kirihiko is just a good Memory salesman. If Darth Vader had a daughter he actually liked and doted on and a pretty decent officer (sorta guy Vader won't hesitate to strangle) married her, if the officer thought he'd be her equal, he's Too Dumb to Live. If you're brave enough, it's still quite a desirable position - fifth-highest member of the organization. And if he hadn't acted respectful, there is no way he would have had a chance of getting that high in the first place.
Shotaro's fashion choice might turn off some Western fans, due to the infamous "fedora stigma" in the west recently.note Fedora-wearers are a rough equivalent of Japanese Otaku regardless of hobbies, often with misogyny and poor hygiene as part of the stigma. Said stigma is not common to Japan, fortunately.
The Woobie: Akiko, especially after she finds out why her father could never see her again in Core.