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.
American Kirby Is Hardcore: The Japanese cover of the first volume has Plue eating a candy with several characters beside him. The North American and European box art has Haru wielding the Ten Commandments. Almost the opposite of what happened with the anime.
Ass Pull: Niebble. He isn't referenced once until Sieg needs help breaking out of prison, and is treated by Sieg as a regular character we should already know.
Ogre is a psychopathic brute defined by his violence, lust and thirst for power. He steals a giant battleship called the Silver Ray, destroying the reputation of its creator and using it to try to annihilate an entire continent. The hot-tempered Ogre will frequently fly into a rage and go on killing sprees of anyone in sight, as well as sacrifice his minions without a second thought. Additionally, he plans to turn every women in the world into a sex slave, as well as kidnaps Elie to rape her.
Shakuma Raregroove is the former sorcerer-king of Raregroove and by far the most evil member of his family. He started a war with the kingdom of Symphonia purely for fun, destroying both countries and all their citizens. He killed Haru's grandfather, the king of Symphonia, with a deadly curse. He abondoned his son (King) Gale so he would spend his life enduring so much suffering until he became a world-destroying nihilist. He allows his magical student Haja to initially believe that he merely wants to take over the world, before killing him and revealing that his true plan is to destroy all of reality out of nothing more than hatred and sadism.
Evil Is Sexy: Reina, although she turns out to be not so evil.
Jiero, Lillith and Leopard play this straight though.
Fridge Brilliance: The Dark Bring corrupt people and turn them into destruction crazed maniacs because they're part of Endless, who's sole purpose is to destroy the world.
Raregroove can also be transliterated as Realglobe.
Remember when Haru was defeated by Oracion Six and tried to protect Elie with his own flesh? It was identical to the death of Haru's mother.
Fridge Logic: How is it possible that Shiba survived an explosion that destroyed one-tenth of the world, when he was right at the epicenter?
The Overdrive wasn't as powerful as everyone says it is. And as for how he survived being in the middle of it, that's hardly the single most fatal thing a main character has survived in this series. If anything, Shiba's survival at the very beginning sets the tone.
The very first chapter shows Plue saved Shiba using the combined powers of the Rave (or something).
Here's a better one. How is it that in the year 0015 about half of the world (at best) was known, causing that 'destroyed 1/10 of the planet' myth listed above. But in the year 0066 they not only discovered the rest of the world and built thriving cities all over it but developed highly advance air ships?
Moral Event Horizon: Played with. King normally would've crossed it when he killed Sakuya, Haru's mom and Gale's wife by proxy and made him into a bomb so he wouldn't ever be near his family; but since this came right after his wife and his son (who was later imprisoned for ten years) was gunned down by an ambitious officer hired by his best friend. It makes it all the more tragic.
The one who really crosses it is Slade, a soldier from The Empire who has King's wife gunned down and then has his son imprisoned for ten years for being related to his father under the pretense he was dead despite Gale specifically telling him not to, all for a promotion. Despite being a minor character in the long run, it makes you consider a genocidal terrorist leader bent on making a former friend's life a living hell the lesser evil in the latter's flashback.
Doryu when he kills Lilith. But to be fair, he warned her first.
Ogre when he reveals that he ruined Reina's life by having his theft of Silver Ray be placed on her father.
The Blue Guardians when they torture Nagisa.
Shakuma is shown to be a horrible person when he kills Jegan just when he redeemed himself and destroys Sieg's skeleton, but truly crosses it when he does three things: 1.) He was the one who started the war 50 years ago For the Evulz. 2.) When he had the King of Symphonia and Haru's grandfather be cursed to a slow death while deeply enjoying it and 3.) When he is revealed to be King's father and admits that he abandoned him without any care for his well being despite his immense powers.
Narm: We get it Doryu, you're the dark lord of darkity darkness who hates humanity and wants to destroy the light. Now shut up and fight Haru already!
The Art Evolution that happened in the Blue Guardians arc that it very similar to One Piece in style has a lot of extremely, ridiculously exaggerated expressions in people's faces that destroys some of the dramatic impact of the story.
Sieg Hart uses this on his enemies to send them to a nightmarish world. When Haru got trapped in it, he watched his mother set herself on fire (which burned him as well) and his sister slit her wrists so she could "give him something to drink".
Also when Deep Snow uses his Dark Bring/Shadow Stone to contort people.
The Producer Thinks of Everything: The first volume's cover already had King and Lucia. Futhermore, in the first chapter Shiba wonders where he first heard Haru's name. Thirty volumes later... Haru travels to the past and Elie yells his name in front of a young Shiba.
The Palace Guardians (Let excluded), Orochi and the Five Demon Lords, especially Ashua, the holder of the fifth Sinclair piece don't do much much other than fight the enemies with little to no backstory at all
Villain Decay: Lucia started off as a major threatening force in his first appearance, with further cameos building up his level of threat, then cue the results of his first real fight with Haru, in which he both loses and becomes pitied. Thus, when when he finally decides to change his motives from world domination to universal destruction due to the fact he lost his mommy, he becomes less a threat and more of an annoyance. Even his One-Winged Angel isn't nearly as scary as his dad's.
Woolseyism: Even if this is just a change from manga to anime, it still worked. The "Dark Bring" were called the "Shadow Stones", "Demon Card" was called the "Shadow Guard", and the "Sword of Ten Powers" was called the "Decaforce Sword". They actually roll off the tongue a little better.
Except for the fact that there was actually a reason behind Demon Card's name, and that the insignia the DC members wear is actually a stylistic version of the DC initials. Shadow Guard still works, but it detracts a bit from it's founder's past.
Also the Ten Commandments > the Ten Powers. Less grand, but more fitting.