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.
Breather Boss: Surprisingly it's the Bonus Boss, Guardian Of Worlds. The only reason being that it doesn't have the cinematic attack thatall the previous bosses have. He does get stronger in subsequent fights though, and his Great Divider attack can instantly kill weak party members.
Demonic Spiders: Anything with One-Hit Kill attacks (such as Darkness Beckons), given Instant Death-protecting gear is VERY hard to come by. They show up pretty often in the game's last few dungeons too.
Ear Worm: The victory theme. You're going to be hearing it a lot.
The World Map theme. You will be humming it before the game is even over.
Game Breaker: A lot of players agree that the Mighty Glacier familiar Dinceros (and its evolved forms) is one of the most powerful familiars to get in this game. Moves pretty slow and its accuracy is just downright horrible, but if you let it learn Earthsplitter and Belly Buster (which are always accurate), watch as hordes of Mooks go down and boss fights become a lot more bearable thanks to its natural high defense. Good luck getting them though.
The casino is this if you're looking to make a lot of money. Simply spend time at the Platoon (while there are times the Random Number God screws you, for the most part it's strategy), exchange the coins for the Great Sage Secret, sell it (they're worth 6000 a pop), and you'll have more money than what you'll need for the entire game. And the Great Sage Secret fully restores an ally's HP and MP so you wouldn't have to worry about healing for quite some time.
The Metal Slime enemies in this game. Though rare and hard to take down with an early-game party, Save Scumming can help that, and the EXP discrepancy between them and regular enemies is just ridiculous. To put it in perspective, the weakest and earliest-available one, Toko, gives out more EXP than most major storyline bosses until about 2/3 into the game. And by the time bosses start giving more, you have access to another variety of Toko that literally gives Over 9000 EXP. (when most other enemies in that area give 350 at best) Even one hour of grinding against these things can make the main storyline an absolute joke.
Genius Bonus: Shadar is the name of a star in the Cassiopeia constellation. Make of that what you will.
In the Glistering Grotto it is possible to be knocked outside the combat stage and off the cliff. The moment you fall through the bottom of the map you teleport to its very top — right over the exact spot your last fall ended. You can loop Portal style for as long as you like, as the lack of air physics means ending the cycle is as simple as walking forward.
A possible Swift Solutions reward is a jump button that is utterly useless while exploring but usable even in combat. It still does nothing... unless your playing as as the comically slow Tin-Man, in which case the jump button becomes a "Move twice as fast" button, completely removing the one Fatal Flaw they inserted to balance his incredible defense and well-above-average strength.
The trick actually works with anything that has a move speed of two or less stars, even Dinoceros.
Internet Backdraft: The North American release of the Wizard Edition could only be received by ordering directly through Namco-Bandai. This turned into a nightmare as the distribution company, Digital River, oversold copies due to a glitch in their ordering system - and then on release day, they unceremoniously cancelled hundreds of orders without warning. What few copies there were in stock were sent to randomly-chosen customers, so that even people who pre-ordered seven months prior were not guaranteed copies. Those who had their orders cancelled were reimbursed with a 20-USD coupon for use in NAMCO Bandai's online store and a voucher for an unofficial strategy guide for a game they don't have, distribution of both also handled by Digital River. Meanwhile, eBay reseller PlayCanada successfully got a hold of over 200 pre-ordered Wizard Editions and scalped them for over 300 USD. On top of that? Some people didn't even receive their coupons.
Monde Green: Sometimes Oliver will shout NEAT-O! after winning a battle, but the way he shouts it makes it sound like KNEEL!
Moral Event Horizon: It was always clear Shadar was not a nice guy, but you truly start to hate him after finding out what he did to Rashaad and Esther. Basically, he forced Rashaad into being a Retired Badass by doing the following: first, he steals his daughter's courage and turns her into a Brokenhearted. Now the guy can't bring himself to do anything for fear of her being hurt further. But it doesn't stop there. To ensure her heart is not just broken, but outright closed, he goes after Esther's Alternate Self Myrtle in the Otherworld. He Mind Rapes her father (also Rashaad's Sould Mate) into becoming a Domestic Abuser, which traumatises her so much she becomes a Hikikomori. All this to elimanate someone who could possibly become a threat to him later.
Nightmare Fuel: The backstory of Xanadu. Shadar had attacked and when he stole a piece of the people's hearts, they turned against one another and killed each other. Sure, there were survivors who fled to a town. To ensure that Shadar doesn't go after them, the survivors turn the townsfolk and any old-enough children born in that town into Brokenhearted. Put it this way: they'd rather Mind Rape themselves than risk Shadar doing it to them instead. Dark stuff.
Shadar in general is this, really. He's an incredibly powerful Evil Sorceror who's The Dreaded just about everywhere, for good reason: this guy can essentially Mind Rape you whenever he feels like it. He's Dangerously Genre Savvy about it too, using his power to remove Enthusiasm and Courage to ensure any attempt at La Résistance is nipped in the bud before it can even begin. And if you're a real threat, he'll make sure you never stand up to him again by turning your loved ones against you. He's also not above going after your Soul Mate in the Otherworld too. Yeah, this guy is basically Paranoia Fuel personified.
The Ashes event. Seeing innocent townspeople who you once knew and helped out turn into mindless zombies who are now out for blood is pretty scary. Saying it's just a Mood Whiplash would be an understatement.
A lot of the zombified townspeople look pretty horrifying too. And the zombified King Tom is surprisingly intimidating.
The Zodiarchy looks incredibly unsettling. Pulsing veins, jittery movements, eyes spinning in the sockets and that freaky mask moving like a demented puppet. You're motivated to kill it as fast as you can just so you don't have to look at the twisted thing anymore.
The whole scene with Rusty is terrifying, in particular to any unfortunate enough to have dealt with a Domestic Abuser. Up to this point, the Brokenhearted you've met have lost their enthusiasm. The result is basically Apathy Syndrome: annoying but manageable. Then you meet Rusty, who's had his Kindness stolen from him, and you learn just how nasty the Brokenhearted can get. Special mention goes to the part right after he hits his wife: he screams at Oliver to leave, and when the camera turns back to him...brr...the (really big, really strong) man is seething, and he has a wrench clenched in his hand...the guy looked seconds away from snapping violently. One can only wonder what would have happened if the Nightmare's emergence hadn't knocked him out...
Drippy's advice can sometimes invoke this trope. It's also not uncommon for him to stop a boss fight to give a hint in the middle of one of your spells or skills, thus negating your attack and wasting your magic.
The tutorials as well. You realize how bad it's going to be when you've been walking around and exploring for about half an hour when you suddenly get a tutorial telling you how to walk.
Pea's dialogue in general once you bring her to the Otherworld. But especially her mispronunciation of 'Sanctify'.
That One Achievement: The 120 Alchemy Trophy and the 250 Familiar trophy. The first one requires you to make 120 different items through Alchemy, which wouldn't be so hard if the drop rates for most items weren't so low. The second one requires you to tame 250 different familiars, and (you guessed it), the capture rates for all familiars are dangerously low. Luckily, you can tame a Tier 1 familiar, evolve it and each form will count towards the trophy so long as you did not catch it beforehand.
That One Boss: The Royal Jelly, a Flunky Boss who swarms you with Jelly Babies with its Baby Shower attack, which can very easily overwhelm you (Thankfully, it stops attacking for a few seconds after it does this). Add the fact that Herd Hitting Attacks are in short supply at this point and... yeah. Thankfully, Drippy's Mother gives you several spells for defeating it.
Before that, the first battle with Shadar on the boat can be this or a Wake-Up Call Boss. He has many spells in his arsenal that hits your entire party for about a quarter of their health and he can cast them in rapid succession. While you only have to take him down to half of his health, its not rare for players to trip up on him.
Khulan's Nightmare can be a real nightmare. Not only does it inflict status ailments such as Poison on you, it spams its ultimate attack over and over. It also hits hard and will trip up lower leveled players if you aren't careful.
Porco Grosso has very high defense against physical attacks, though thankfully it happens to be weak to electrical attacks otherwise. It also happens to be very agile as well.
Bonding with Esther and Swaine outside the main quest falls mostly away. Most of the time only Drippy has a comment to make, while the other two are suspiciously, and sadly, silent even in moments where you would expect them to say something, especially Swaine.
The games establish that multiple versions of universes exist, not only Oliver's and the Other World from Jet-Black Mage, but the world where the Conductor lives, their parallels from The White Witch and many more. The two Other Worlds are remarkly different in locations, characters and events, yet only vague references in the different universes point across to each other. The concept is only plot-wise remarked upon in The White Witch through optional bosses that are stronger alternate versions of bosses you fought in the main quest and which have breached through from parallel universes. Once they are defeated, you can take on the Guardian of Worlds, a creature that stands in the center of all possible worlds.
Values Dissonance: The game got an 18+ rating in South Korea because of gambling that can be done (not even with real money).
Westerners Love the White Witch: While its sales numbers do not diverge widely between countries (due to the "children's JRPG stigma", you could say), Western players and critics are much more enamored with White Witch than Japan. There are two reasons for that: first, Eastern RPGs are becoming steadily more scarce in the West, especially on consoles, so a big-budgeted one for the PS3 like Ni no Kuni draws attention. Secondly, and more importantly, Japan was "burned" by Jet-Black Mage back when it was released. Level-5 promised that the two games would be widely different and also complementary to one another, but they only had very minimal differences in terms of plot and gameplay (putting it another way, they were promised Persona 2, but got Pokémon Crystal). Those who bought the original (and they were many, it sold really well) didn't want to buy the same game twice, and critics couldn't quite "forgive" it. However, since that version was never released in the West, White Witch became much more unique in Westerners' eyes and could be judged (and sold) as its own, standalone title. It seems to have paid off.