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: Hatoful Boyfriend
Accidental Innuendo: One particular item in BBL is referred to as a Bright and Massive Thing. Among others, this leads to the line, "You're up, Massive Thing!"
It's actually referred to as the Massive Key, designed to open any lock. (Personal theory: it's a crowbar.) Ryouta's the only one to call it a "Massive Thing."
Also what Ryouta says after being knocked out.
Ryouta: "Aargh! Darn it! Why did I let myself get taken from behind like that!?"
Complete Monster: Doctor Shuu. Oh, man... If you take his route, he sends you a quill pen and a roast, both of which are originally Yuuya, and then in the end, he kills you. He is also the one responsible for Ryouta's weak stomach, weakening his immunities on purpose to make him a carrier for a human-eradicating virus.
"Ousama" for The King. "Ousama" is what he calls himself in Japanese, and it does mean basically the same thing.
Basically, anyone who regularly plays this game, especially with friends, develops nickname. Examples: "Sadbird" for Nageki, "Saddest Bird" for The King, "Fab Bird" for Nishikikouji, "Secret Bird" for Anghel, and "Unapologetically Racist Bird" for Sakuya.
Heroic Sacrifice: Nageki kills himself in an attempt to destroy the Charon virus, so that he can't be used as a weapon against humanity.
Genius Bonus: Many of these may be apparent to players with hobbies related to birds, such as keeping pet birds or studying ornithology. See the Hatoful Boyfriend Fridge Brilliance section for a more complete and detailed list of these.
Upon introducing Shuu, Hiyoko mentions that even just talking to him can bring you down with horrible diseases such as Aspergillus or Psittacosis. Both of these diseases have certain strains that can affect both birds and humans.
Ho Yay: Mainly in the Bad Boys Love route, although it's limited to Doctor Shuu's unrequited feelings for Ryouta's father in a flashback, and possibly Kazuaki's feelings over his late foster-brother Nageki.
Yuuya's flirtatious nature is by no means limited to girls. He even says, "I'm always straight in love. Well, almost always." When he overhears Ryouta and Sakuya discussing him, he says, "I guess I'm topic of choice among the boys now as well as the girls. Not that I mind."
Note: Sakuya is his brother.
The manga also alludes to this when, after Ryouta tells how Yuuya's locker is usually overflowing with gifted beans on Legumentine's, Sakuya mentions that St. Pigeonation's is (excluding the player's character) an all-boys academy.
In Holiday Star, Hitori and the original Kazuaki seem to have been very close, albeit in a horrendously dysfunctional way, with Hitori ultimately taking advantage of Kazuaki's trust to kill him and steal his identity. In the alternate universe shown in the sidestories, both of them are alive and working together at St. Pigeonation's, and Kazuaki is still very attached to and dependent on Hitori, who seems rather less enthused about the whole affair.
Les Yay: Hiyoko's admiration of Azami borders on full-blown swooning in a couple of scenes with her even thinking, "She's so cool! I think I'm in love...!" after Azami saves her from a gang of punkgeons. Sadly, there's no actual route for her and her side story consists of Hiyoko setting her back up with her former boyfriend.
It Was His Sled: The manga tells you right from the start that Nageki is a ghost, which was only revealed at the very end of his route in the game.
For a while, due to a translation error accidentally saying Dr. Shuu had hands, there was a minor meme in which Shuu would be drawn or Photoshopped as a bird with human arms and hands sticking out of his body.
In Holiday Star The King is original Kazuaki's witch form after he fell into despair, and Holiday Star is his witch's barrier.
Still a better dove story than Twilight.
MST3K Mantra: This is how Ryouta explains the congruency between BBL's and Holiday Star's timelines during Holiday Star's Radio Session. "Just don't think about it too hard and you'll be fine! In the end, it's because unlike books or movies, visual novels have all kinds of branches and possibilities."
Narm Charm: The premise is ridiculous, the script is riddled with typos and the occasional formatting error, the graphics are a somewhat jarring mix of bird photos, 3D backgrounds and the occasional hand-drawn or photoshopped image... and somehow, it all still works.
Nightmare Fuel: The Hatoful Boyfriend series contains enough Nightmare Fuel to warrant its own page.
The scarecrow-like monster in the Bad Boys Love route. You will jump when you first see it. So much more nightmarish when you learn its origins.
There's little of BBL that doesn't fall under this trope, and most of what does falls under a much more depressing category. The humorous bits that would be perfectly at-home in the normal routes actually come off as being completely out of place at this point.
The Game Over screen when you get killed at your home. It looks like something out of Yume Nikki.
Hell, the entirety of the 4th chapter. If you make all the right choices, it's horrifying enough. Make a wrong one at any point? Forget about sleeping that night, and good luck at ever taking the phrase "Happily Ever After" pleasantly ever again. It even makes a freaking picture book terrifying.
Surprise Creepy: Again, a game about dating pigeons has its own Nightmare Fuel page.
What Do You Mean, It's Not for Kids?: The English download site for the full game gives it a G-rating. If you've played this game in its entirety, you'll know just how deeply and hilariously wrong this rating is.
What Do You Mean, It Wasn't Made on Drugs?: Well, it did start out as an April Fool's joke...that being said, Oko San and Anghel's routes (and the characters themselves) are exceedingly bizarre even for this game.
Actually, subverted in that, despite the things appearing completely and utterly crazy in the beginning, all this has an explanation.
In Holiday StarThe King becomes this once we learn of his tragic back story. His back story may especially hit close to home for players who have ever suffered from social isolation, clinical depression, and/or suicidal thoughts.