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.
YMMV: New 52
Author's Saving Throw: Martian Manhunter told off his Stormwatch teammates for referring to him as a Justice League member, which is apparently shorthand for "established superhero", despite the fact that the Justice League has never had more than seven members (until JLI). So, any author wants to refer to, say, Metamorpho or Green Arrow as "Justice League heroes", that's what it refers to.
Broken Base/Internet Backdraft: The reboot in general was very controversial when it was announced, and even two years after it happened opinion was divided and discussions could get very heated about certain aspects of it. Overall, some fans consider it an interesting and fresh new take on the universe, while others consider it the return of some of the worst tropes from The Dark Age of Comic Books.
Superman is introduced as a more angry brooding figure considerably more violent than any previous incarnation being willing to throw defenseless white collar criminals out of windows and dangle them off ledges, mocking the police in his early twenties and choke slamming Batman in their first encounter.
And of course, the Executive Meddling is still going strong. Some fans are upset that the comics appear to be even more editorial driven than before the reboot.
The disappearance of long time fan favorites such as Wally West (the third Flash - along with his wife and kids), Cassandra Cain (Batgirl II/Black Bat), and Stephanie Brown (Spoiler/Robin/Batgirl III/etc.) among others while supposedly "inferior" or less beloved characters such as Bart Allen remain has not sat well with many fans.
Reaction to Villains Month was, for the most part, lukewarm at best. Grievances range from normal issues of the entire line of books being put on hold for a month, to the stories being boring and/or uninspired, to the 3D covers being an excessive gimmick no one particularly wanted and unnecessarily driving up cost.
Also the decision to update the Justice League's roster, first by removing Firestorm and Element Woman after they barely did anything since joining the team, the addition of Shazam (with fans preferring he and his foster family get their own book), and most of all, removing The Flash and Superman and replacing them with Captain Cold and Lex Luthor respectively.
The Joker in the storyline Death Of The Family is as monstrous as ever. Starting by attacking the Gotham City Police Department to retrieve the face he had cut off, he murders nineteen police officers and threatens to kill Mayor Hady, but instead kills Hady's police guards. Reenacting old crimes, the Joker attempts to poison Commissioner Gordon and blows up the Gotham reservoir. The Joker hunts down the whole Bat-Family, trapping them in the Batcave and leaving them soaked in gasoline while he forces Batman to watch. He attempts to set them ablaze before Batman stops him, but leaves a contingency: his special Joker gas to force them to kill one another if not for their Heroic Willpower. The Joker here shows how terrifying he can be when he decides to cut loose and is defined by his twisted obsession with and desire to hurt Batman.
Batwoman: Maro purposefully drowned the children of Maria, the woman who would go on to kill herself and become the Weeping Woman, because his magic relies on the power of belief. Because people believe in the legend of the Weeping Woman, Maro intended to use that to keep Maria from passing on after she inevitably killed herself and transform her into the urban legend, and she would go on to steal and/or kill more children.
Swamp Thing: Anton Arcane's new incarnation is a being of boundless cruelty. In his pursuit of immortality, Arcane took over the elemental force of The Rot, the natural force of death and decay, and subsumed it to his will. Arcane extends the Rot to devour all that lives and trap the world in deathless limbo he can rule. Arcane even forced his own nephew to be the Rot's avatar, devouring his mind from within. In Arcane's battles with Swamp Thing, Alec Holland saw a future where Arcane gruesomely tortured and beheaded Abigail and saw lobotomized clones Arcane had made of her as cannon fodder.
In the New 52 Universe, Mongul is as brutal as ever. When he sends his War World battle station to conquer a planet, the Admiral in charge of the planet's defenses demands his surrender. Mongul kidnaps the Admiral and gives him tour of his space station. Mongul shows him the Gladiator Games, where the loser of the match will have his entire race exterminated. Mongul then reveals that he killed his own brother and parents to gain total dominion over War World. Mongul then shows the Admiral his collection of Black Mercy plants that he uses on victims to make their greatest fantasies come true or make them feel their greatest fears. Mongul also reveals that while they were talking, War World has destroyed most of his planet and the survivors will taken as slaves. Those too weak to be slaves will be have their organs harvested. Mongul then reveals then final room in his gruesome tour, a room full of heads on pikes. When the admiral asks what was their crime, Mongul states they had the gall to demand his surrender. He then rips off the admiral's head and places it on a pike.
"The Pearl" (which ended with Damian's death and members of the Bat Family mourning him-plus Batman getting another Wangst period during which he gave the most violent Never Be a Hero speech to someone ever).
Ensemble Darkhorse: Despite how divided the fandom is with the whole reboot in general, some characters are quite well liked for their interesting characterizations and potential for stories, such as Aquaman, Strix from Birds of Prey, and Billy Batson's five foster siblings.
A common gripe with the New 52 is that despite a large number of changes, the Continuity Lock-Out that was typical for the old DCU did not disappear, and many characters (such as Green Lanterns) still require extensive knowledge of their old DCU stories to understand what is going on, instead of having a fresh start. What makes things even more confusing is that many older fans who would be otherwise immune to Continuity Lockout are confused as well, due to the fact that whatever continuity that has been retained has been very hazily defined. For example, according to the new "superheroes debuted five years ago" compressed timeline, Batman started his career and took on all four male Robins within six years.
Earth 2 has received this kind of complaint, since it eventually turned from a different team of superheroes similar to those from Justice Society of America fighting against the onslaughts of new villains and a different member of the forces of Darkseid to Batman's team vs. Superman's team.
The same can be said of the practice as of July 2014 to re-relaunch cancelled series under new creative teams, when one of the points made in the New 52 is that there are only 52 running books at once, taking space from newer ideas in an attempt to reboot a reboot.
Tainted By The First Issue: Since DC released fifty-two first issues in one month, even people willing to buy all fifty-two #1s were unlikely to give anything they didn't like a second chance. (How many #2s are you going to buy of comics you already have a bad impression of? When there might be dozens of them?) Ergo, any writer who put something controversial in the first issue, expecting readers to stick around for justification in later issues, was making a grave mistake. Several of the series died due to their first issue not being as good as later ones.
However, the "add something controversial to hook in fans" thing was a double edged sword. Catwoman and Red Hood and the Outlaws drew in massive amounts of outcry and drama over scenes of sexualization (Catwoman having rough sex with a hesitant Batman, Starfire in some cheesecake poses and having sex with Roy Harper with the then perceived notion that she couldn't tell humans apart). How the writers responded to the allegations of angered fans differed: Judd Winick didn't really do much to defend himself and was replaced rather quickly, while Scott Lobdell openly responded to fans and lasted much longer on his book.
Superman was reintroduced as a man who in his early twenties was violent towards non powered criminals, openly mocking the police as he ran from them and a few years later in the timeline, choke slams Batman in a fight when he already knew nothing in Batman's arsenal could hurt him. Even the more grown up version introduced in issue one of a parallel series was shown to be a more angry brooding figure than Superman had ever been.
One of the reasons for this reaction was that quite a few titles suffered from an Aborted Arc in their cancellations - although most of them wrapped up fairly smoothly, there was still a small forest of dangling plot threads left behind, presumably now never to be resolved.
One of the most controversial examples of this in fandom is giving The Phantom Stranger a definitive origin.