Ass Pull: Amren coming Back from the Dead, memories completely intact, at the end of A Court of Wings and Ruin. Not only does it directly go against what she'd repeatedly stated—that if she regained her true form, her memories would be gone forever—there's absolutely no foreshadowing, and the explanation that "a piece of her was stuck in the Cauldron" is pretty unconvincing.
Tamlin is the most divisive character in this series. Ever since his actions in A Court of Mist and Fury, fans of the series have been split up into those who hate him with a passion and those who think that the author has gone too far with vilifying him to the extreme. Those who hate him dismiss the idea that he was ever a good person to begin with and that in A Court of Mist and Fury he showed his true colors; those who sympathize with him bring up the many layers of characterization and his Hidden Depths as justification.
Rhysand. Critics accuse him of being just as bad as Tamilin, if not worse, for his villainous treatment of Feyre Under The Mountain and tendency to manipulate and/or lie to everyone, even those close to him. Fans point out that unlike Tamlin, his treatment of Feyre was while he was a villain, whereas Tamlin was her fiance, and like him for growing kinder and trying to help Feyre overcome her PTSD.
Feyre is not without her detractors. There are those who think her jaded personality from the start was a Cliché Storm, those who like her as a human but think she became less interesting after she became a High Fae, and those who appreciate her character arc in its entirety.
Creator's Pet: Rhysand. He goes from being The Dragon to Amarantha's Big Bad in book one to the main love interest, Feyre's Soulmate, and has his formerly evil traits almost fully removed and replaced with positive ones. Between the constant descriptions of his handsomeness, his status as the most powerful High Lord, and Common Mary Sue Traits, it's not hard to see who Maas's favorite character is.
Ensemble Dark Horse: The secondary characters are quite commonly praised for being very interesting and nuanced, some moreso than others.
Tarquin, for being the genuinely nicest High Lord with a good amount of common sense and having dreams of overturning the racist aspects of Prythian culture.
Karma Houdini: Feyre never suffers the repercussions for having destroyed the Spring Court, facilitating the advance of Hybern's forces through Tamlin's territory and eliminating a potential ally for their efforts after she succeeded in her revenge plan. It is implied that the civil war she sparked may have caused the deaths of thousands of innocents and yet she receives no comeuppance for this. Worse still is that as of A Court of Frost and Starlight she lives a privileged and happy life, unfazed by any sense of guilt or remorse for her actions.
Some of the sex scenes in A Court of Mist and Fury. Things such as Feyre's vagina glowing, and Rhysand's climax shattering a mountain.
The term "vulgar gesture" is used often in the books. However, it's never once described what this gesture looks like and why it's so vulgar. The most anyone can figure is that it's simply Flipping the Bird. Which begs the question of why it's never described as such from a book that throws around "pissy" and "prick" just as often.
The constant uses of "male" and "female" to refer to the gender of a character has been mocked, alongside the sheer number of times "mate" appears from Feyre realizing Rhys is her mate on.
Never Live It Down: Many readers believe that Tamlin's lowest moments i.e. locking Feyre up in his manor and allying with Hybern, define him as a character and previous characterization present in A Court of Thorns and Roses was just a facade.
Protection from Editors: Many readers have picked up on the frequent misuse of em-dashes and ellipses, grammar mistakes, as well as anachronisms - the usage of "pissy" or "pissed off" in a supposedly early modern world, the existence of flushing toilets and modern stoves paired with the lack of crossbows or firearms and the usage of bows for hunting.
Romantic Plot Tumor: It's argued that the Amarantha plotline and the war with Hybern are put on the back-burn for Feyre's relationship with Tamlin and later Rhysand during the first two books. The third arguably alleviates this, however, by being more focused on the war efforts.
Sequelitis Probably the best way to describe the reception to A Court of Frost and Starlight. Many fans found it to be a very pointless installment for something that was meant to "bridge" the two series together. Characters felt very out of character, Feyre stays at home while Rhysand does all the political work (which was a major issue with Tamlin). Regardless of how people feel about Tamlin, most agree that the plotline is done yet it's still apparently continuing. The reception for this novella got so bad that some of the more hardcore fans confessed to being tired of Feyre and Rhysand.
They Wasted a Perfectly Good Character: Prince Dagdan and Princess Brannah, the creepy twin emissaries from Hybern. They show a lot of political cunning and power, engaging in literal mind-games with Feyre and playing her as good as she plays them. Their presence also has the possibility of giving first-hand insight into Hybern culture. Too bad they're killed off a third of the way through A Court of Wings and Ruin
Amren. She's not a fae, but is instead an otherworldly being heavily hinted to be an angel of death. She's Small But Fierce, drinks blood, broke out of a deadly prison, and... sheds her otherworldly form to become a High Fae, and that's the last we hear of her abilities.