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.
All There Is to Know About "The Crying Game": Go into a Worm thread on a forum somewhere and see how long you can go before someone casually spoils any of the major twists, like Behemoth's destruction leading to the emergence of three more Endbringers, Dr. Manton being responsible for the Siberian or Scion being the final villain. Go on. We'll wait.
Marquis' involvement with Iron Rain, and Jack Slash's assertion that he did not kill her, have lead some readers to speculate that he did not kill Iron Rain, but blamed himself for her death. (Interestingly, Panacea's mother has never been named. Some speculate that Iron Rain was Panacea's mother, and that Marquis blamed himself for her death, possibly at Allfather's behest.)
Some readers believe with varying levels of seriousness that the mysterious Sleeper's power isn't really some Story-Breaker PowerPerson of Mass Destruction thing, but rather the ability to make others think he's that powerful.
Really, the story is filled with this. A common comment on Wildbow's writing is that he rarely ascribes purely negative or purely positive qualities to his characters, making their moral position whatever the reader ascribes to them.
Taylor's actions in the penultimate chapter. Did they count as bullying or not? If so, were they justified given the threat humanity faced? (And the billions said threat had already slaughtered?) Has Taylor fully transitioned into a monster like those who tormented her, or were her actions a necessary evil against a threat that could be stopped no other way? All of these questions are brought up in-universe in the final chapter.
Complete Monster: Jack Slash, full stop. The man's spent his life murdering his way across the country, and his personal bodycount has to be in the hundreds at least. That's not to mention everything he leads the Slaughterhouse Nine in doing, like attacking a hospital and targeting the maternity ward first. He eventually embraces his role as the catalyst for the end of the world because he wants to be remembered for generations as one of history's greatest monsters.
Toward the latter story arcs (e.g. when a new Endbringer designed to counter what they used to kill Behemoth appears), some readers have fallen into this state on the grounds that humanity is simply too overmatched by the evils that assault it. And this is before Scion decides to start annihilating everything in its path.
There are distressingly few true heroes in the setting, with almost all the rest being people who are at best high in the "hero" scale of Anti-Hero (and can still be pretty lousy human beings, just falling under the In-Universe classification of "hero") and at worst people who get put through the wringer because this setting LOVES No Good Deed Goes Unpunished.
Ensemble Darkhorse: The Slaughterhouse 9 and the original 3 Endbringers are the story's iconic villains and tend to overshadow all of the others. Among the Endbringers The Simurgh is generally viewed as the coolest/scariest.
Fan-Preferred Couple: Almost any chapter with both Taylor (Skitter) and Dennis (Clockblocker) will have at least one, and probably several, comments to the effect that they should hook up.
Lampshaded, post- Time Skip Clockblocker complains that all the rumors about him having a crush on Weaver have put a major damper on his career.
Knowing what is going to happen to the two sisters makes Glory Girl's interlude much tougher/sadder to read.
The reveal about just what Noelle can do changes how you read The Extermination Arc and Dinah's Interlude. Theoretically, the world's end almost started right there!
In Drone 23.5 it's joked that Taylor's lovey-dovey speech to the Undersiders will get her killed. But it's not her who bites it in the subsequent Behemoth fight...
He's Just Hiding: Some fans reactions to the news of Battery’s death. Also, nearly everyone's reaction to Clockblocker's... disappearance. Though the latter character did survive. Sort of.
Hype Backlash: Some of the readers coming in after the story's completion are scrutinising it with a more critical eye than the readers there while it was in progress, and not all of them agree with how things pan out. The darkness of the Crapsack World is a common contention.
I Am Not Shazam: The title Worm seems to refer to Taylor/Skitter's abilities to control bugs, supported by a minor villain even referring to her as "the worm" at one point in the narrative. In actuality, the 'Worms' refers to the reality-spanning entities (who create/use wormholes hence the name) that are the source of parahuman abilities... and are Omnicidal Maniacs to boot.
Inferred Holocaust: While the numbers aren't given in-story, Behemoth's attack on New Delhi probably killed millions of people.
Scratch "inferred"; Behemoth levels at least half the city (which has a population of 10 million), and it's explicitly stated that the majority of the people there did not get out in time.
Karma Houdini: The Number Man. All of the other Cauldron Capes ultimately suffer some kind of comeuppance for all the reprehensible things they've done. He never gets so much as a Heel Realization despite the fact that he's arguably the worst of the lot.
Kick The Bitch: The cold, systematic way Shadow Stalker is forced to watch as her life is destroyed is utterly horrifying - but she's such a vicious, bullying sociopath most readers agreed she had it coming.
Les Yay: Bitch sleeps cuddled up with one of her female minions, there are also several points in the story where Taylor suspects Bitch is about to kiss her.
Magnificent Bastard: The Simurgh. Let's start with the fact that, despite being completely unable to communicate in any meaningful fashion with anyone apart from other Endbringers, she was able to fake a weakness...
All of the new Endbringers that showed up after New Delhi are generally viewed as much lamer than the original three. A big part of this is probably that they didn't get enough screentime to make any sort of impression. Compare Tohu and Bohu being introduced and then largely forgotten about within a single chapter to Leviathan getting an entire arc to himself.
Garrote. A Case-53 whose tentacles instinctively want to strangle any stressor. Points to trying to reach out through the internet to find a friend.
Bitch. Decidedly the Jerkass subclass: she's angry, hostile, anti-social, whatever you want to call it ... but only because she got screwed over by her power and she never had a real friend in her life.
WagTheDog. We knew from Interlude 19 (Donation Bonus #2) that she really wanted to become a minion to Bitch, but when we see her in 21.6, the way she lights up whenever she gets praise from Rachel and the way she sleeps curled up with Rachel ... she clearly came from some terribly-dysfunctional place.
Taylor herself gets this on occasion when life decides to screw her over. She's of the Iron subclass, but it's hard not to feel really, really bad for her past and the general hell she goes through, combined with her self-loathing. Indeed, even many of the people who ascribe to the Alternate Character Interpretation and say she's become a bullying villain herself at the end of the story still find her sympathetic, because she didn't entirely have a choice in what she became.