Arc Fatigue: The mystery about who "D" is lasted from Chapters 18 to 24. "D" was falsely hinted at repeatedly with Draco, Kii, and Lilith. Doppel provided the initial note, while Lala wrote the second and final note.
Mero's talk in Chapter 34 about learning to be more careful with people and trying to make up for her past mistakes seems to be an attempt to rehabilitate her a little from the incident listed under Never Live It Down. Later she got a whole story arc on the subject.
The fact the author has been adding more male characters that are good people and are also in relationships with Extraspecies women to deal with complaints regarding Kimihito's artificially looking better than every male in the story. In fact these guys are in a more loving and stable relationship with these women than he is.
The lamia race's culture of Black Comedy Rape and taking of Sex Slaves is mitigated by info in the omake for volume 7. Namely they don't harm the men and once the deed is done they don't force the men to stay (or at least Miia's father was allowed to run away). Rather the fact that all lamia are beautiful women with incredible sexual skills means that most of the tricked men decide to stay with them. Though that can come across as Stockholm Syndrome.
Less monster girls have been falling in love with Kimihito, most likely as another part addressing the criticism of most Extraspecies women falling in love with him for flimsy reasons. And possibly because his unwanted harem has already reached a size that's difficult to manage.
Chapter 43 shows the girls actually helping around the house and earning extra money to help.
Manga readers didn't like that Polt's introduction chapter was skipped in the anime. An OVA based around that chapter was released in November of 2016.
To balance out the extremely Gonk nature of male centaurs and orcs, attractive looking male liminals have been added to various background scenes, including dwarves, elves, kobolds, and minotaurs (the latter two being attractive Beast Folk).
Asspull: Suu pulling New Powers as the Plot Demands every couple of chapters. She can shapeshift, read minds, taste poisons and so on. Mostly justified due to slimes being a very recent game market invention, as opposed to most of the other monsters who are folklore figure from pre-16th Century times (though Suu still has more abilities than Slimes are ever marketed with at any one time, and some that were never seen on them before her).
Base-Breaking Character: Is Miia really a defrosting, Jerk with a Heart of Gold who tolerates her darling's care for the other women? Or is she secretly a full-blown Yandere who intends to prevent the other girls from getting close to her darling and intends to remove them as romantic rivals. Some theorize that her clinginess may eventually harm the man she loves on a serious level one day... well, more so than now.
Big Lipped Alligator Moment: The OAV has a scene of Polt gently licking Manako's eyeball to remove a hair that was stuck there. It was entirely original, not coming from the manga or any of the omake, and at one point it even switched to Manako's perspective, giving the impression that Polt was licking the screen. Even the other MON members didn't seem quite sure of what was going on. Perhaps some of the animators (or Okayado himself) had this specific fetish...
Fans have been split on the official spelling of the characters' names revealed in chapter 10, especially Meer (German for something between "ocean" and "sea")/Miia. This died down when the manga and the anime both decided to ignore it.
The dislike of the translation choices in the official English manga has caused not so much a split, but a near-unanimous decrying of it. The translation in itself isn't bad or incorrect, it's just filled with a Hurricane of Puns. Which to some people is a lot worse, even though the original Japanese text is chock full of transparent word play as well.
In the official English translation, Rachnera's dialogue is riddled with spider puns and uses "itsy bitsy" as an adjective in nearly every single sentence. Some think it's an amusing touch, while others find it overused and annoying.
Fans who were hoping the anime would get a quality English dub weren't very thrilled when it was announced that Sentai Filmworks got the license to it, since Sentai doesn't have the best track record when it comes to dubbing their shows, with most of them either using amateur voice actors or not having a dub at all. They later announced that the anime would get an English dub, but fans are still uncertain whether the dub will be good or not.
Polt being Demoted to Extra in the anime. The manga chapter that featured her was likely cut out due to space constraints, and as a result she's been relegated to a few cameos. Many of her fans are not happy with this, while others are just glad she's in the anime at all. Her contributions to Kimihito's food problems being cut out of Episode 12 only made her fans even more unhappy. It could have been even worse; she might not have even appeared at all had Okayado himself not requested she be put in somehow. It's mostly died down after the release of the first OVA, which was all about the trip to Polt's gym that was cut out.
Captain Obvious Reveal: Chapter 34 reveals that Mero is an actual princess. Considering the huge and unsubtle amount of foreshadowing, the reactions of most readers was "wait, that's supposed to be a reveal?" The summary blurb on Volume 9 even notes that it's really not much of a surprise.
Counterpart Comparison: Centorea has been compare to Saber from Fate/stay night. This is because both are blondes with a knight's code and who are very protective of their master who they love.
Ear Worm: You'll be forgiven if you can't get the opening song out of your head.
Evil is Sexy: The villainous monster girls are just as attractive as our main characters.
Family-Unfriendly Aesop: Brought to us by Rachnera no less. As Centorea and the audience learns, sometimes it's okay to have feeling of hate for someone and speaking your mind. Because it's better to be brutally honest about your hatred for someone else rather than trying to fake your acceptance of them.
Foe Yay: Rachnera and her love for torturing anybody.
Germans Love David Hasselhoff: The manga is a decent seller in Japan but doesn't make a huge splash. In America, it regularly hit the New York Times Manga Top 10 before the anime was even announced. It's even outsold complete juggernauts like Attack on Titan and even Bleach once or twice.
Papi and Suu experience most of it. Suu notably kisses her outright during the Chapter 10 water-absorption spree, though she does end up bringing Miia, Papi, and Centorea to orgasm.
In Episode 10, Papi sucking on Kii's breasts (to remove toxic nutrients) and Kii sucking on Suu's breasts. (Along with Kimihito replacing a bondage Papi on Kii's breasts, all three girls orgasm.)
Also in Episode 10, Rachnera takes mischevious Lilith to a warehouse for sex and interrogation. Lilith refers to Rachnera as "oneesama" and answers her questions, getting an orgasm as a reward from Rachnera.
The Eel Mermaids get rather touchy-feely with Miia and Centorea, and that's even considering that they're masseuses.
Chapter 1. Miia accidentally dislocates her darling's arm and agent Smith tries snapping it back into its correct position.
The OVA has a brief scene in which Polt licks Manako's eyeball.
For the Western Audience, characters of the same body build as Papi (that is of the young teenage girl variety) getting roped into sexual scenarios. For some, showing said character in sexually implicated scenarios (such as sucking on an ice pop in the most odd of ways) can be a little squicky for those not into the whole sexualized young girls trope. Though thankfully it's not nearly as prevalent as all the other ladies.
Lilith. She only makes one major appearance and a couple of minor ones, but between her prankster personality and her... relationship with "Mistress" Rachera, she had so much more potential for both comedy and characterization.
The extraspecies idol group ANM48 (a parody of AKB48 composed of Little Bit Beastly girls) could have been an interesting way to discuss Fantastic Racism issues, as well as to parody Japan's obsession with idol groups, but they pretty much disappear from the story after the first chapter.
Chapter 14 sets Rachnera up as the manga's first possible villain, yet she was disappointingly added to the harem near-instantly. It would've been more climatic had she been a slightly longer-term antagonist with a sympathetic backstory that became a love interest.
And then there's Draco from chapter 19. Possible rival and villain...and he turns out to not even be an actual dude.
Not to mention the anticlimax resolution of the "D" mystery. The first letter was a joke. The second letter appeared to have been done by yet another goofy girl. The author got quite the knack to drop what could have been interesting plotlines for slapstick characters.
Chapter 26 (the one with Ren Kunanzuki, daughter of Rachnera's previous host family, trying readopt Rachnera out of guilt for her parents selling her) could be quite a heartwarming and well-written resolution for probably the most tragic main character in the manga, and also the debut for at least one sympathetic human person aside of a main cast. Instead, like with almost every other non-monster character, Ren was portrayed as totally egoistic and ignorant, and her appearance was played mainly to make Kimihito look better, having no impact on the overall plot whatsoever.
According to leaked pictures of the new chapters, it seems that every major problem Kimihito and the girls face is going to be resolved in a few pages with no consequences, adding to the light-hearted, comical feel of the series.
Vanilla Protagonist: As with any other harem protagonist, Kimihito is as designed to be an extremely accessible character who can fit any role. Itís the harem girls who provide the true focus of the series.
A funny example. Since Kimihito's death threat, Ms. Smith assigns a member of MON for protective detail during his dates. You'd think Doppel, the shape-shifting infiltrator, would be the perfect candidate for keeping an eye on him incognito; instead, Smith assigns Tio, the dark-skinned, horned giant. There's even a panel where pedestrians look at her in shock while she (futilely) tries to be discreet. Considering Doppel instigated the whole plot, it makes sense she's not available.
In doing this Doppel herself qualifies, as she accomplished the exact opposite of her job. Worse, her prank ended up allowing Lala to send an actual death threat unnoticed, as it was written off initially as another product of Doppel.
When Mero and Kimihito are trapped underwater, and Mero's swimming abilities are hampered by her injuries, it's understandable why the other girls don't jump in themselves to help out: they're not suited for water. But there's no excuse for Octo to just stand there gawking with the rest of them as Mero scrambles to save Kimihito on her own: she could swim just fine. Admittedly, real octopi are fairly weak swimmers with low endurance compared to fish (they can jet away quickly but can't keep swimming for long). On the other hand we aren't told if this applies to Octo and she would still at least be able to effectively walk and cling to the floor, assuming she could reach it before the currents pulled her into the spikes.
Wheelchair Woobie: Averted with Meroune. She is chipper, polite, and only needs the wheelchair when she's not in her room, which has a pool of water to accommodate her. That being said, she's started to rack up Woobie points as of recent chapters: she's realized that a tragic romance may not be all it's cracked up to be after first feeling jealousy toward Miia. From there, she's had to deal with one torturous scenario after another, from the prospect of Kimihito dying to perhaps having to leave him and the rest of the gang forever: ironically, all tragic scenarios that would fit her initial dream.
MON's acronym was changed from "Monsters of a Neuro" to "Monster Ops: Neutralization" in the official English translation, changing a case of Gratuitous English into a name that made sense in English. The anime goes with "MONster's New Law".
Lala, in the English manga, speaks with a notable Irish brogue which fits the Dullahan's origins.