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.
Fridge Brilliance: In the prologue of Dawn, Arikado notes that because Soma wasn't in danger for a long period of time, his Power of Dominance became dormant, causing all of the souls he absorbed to be released. This would explain why enemies killed in Aria (such as Balore) are alive and kicking in Dawn.
Fridge Logic: In part of the True Ending of Aria, Castlevania has recognized Soma as Dracula, due to defeating Graham with Flame Demon, Giant Bat, and Succubus souls equipped, giving him Dracula's characteristic powers. If the Castle has recognized Soma as its ruler, why are the monsters still attacking him? It can't be said that he's not TRULY Dracula, as he's able to enter the one area of the Castle that ONLY Dracula can enter.
A likely explanation is that while he is recognized as Dracula in his ability, his character or "soul" is what prevents the castle from recognizing him as Dracula. So while he can use Dracula's abilities to get into the secret area, what prevents him from commanding the demons is because he still has his human soul.
That, or the castle recognizes Soma as Dracula but the monsters don't.
Not the only one. The Claimh Solais sword, Eversing armor, and souls such as Mantle are ridiculously supercharged.
In Aria of Sorrow, a number of souls have this effect at various points of the game:
The Giant Ghost is the most notorious. Its effect is to deflect incoming projectiles. While this may seem trivial (as it does not stop you from taking contact and melee damage), everything that has a solid marker will be deflected. This includes Manticore's fireballs and Death's mini sickles. Yes, it trivializes many of the enemies that you will face.
The Alura Une/Arc Demon combo allows you to "recycle" your health (instead of losing it) at a 110% percent efficiency, rendering you nearly invincible.
The Red Minotaur can singlehandedly devastate most armies in a single swoop. Yes, it can. Also, facing backwards makes it hit twice. Its MP cost is restrictive, but once you get the Chaos Ring...
Lightning Doll is the universally acknowledged weapon of doom. It is acquired midgame, and has awesome damage for a moderate price tag. And its range? Phenomenal.
The Mimic soul has no combat purposes... But it has the ability to allow you to jump into spikes and gain close to 1000 money in one hit. Do this for a while and you can purchase the Soul Eater ring with ease.
The Mandragora soul in Dawn allows you to throw a Mandragora. The explosion does tremendous damage and has a massive blast radius that deals slash-type damage, which a significant number of foes are vulnerable to, all for 30 MP. It can be acquired early on if you get lucky with the Mandragoras, and is useful from when you get it until the very end.
Abaddon's soul, in Dawn. Though you don't get it until very late of the game, absolutely trivializes any part of the boss rush with a large, slow enemy.
The Persphone soul in Dawn turns dealing with nearly all enemy projectiles into a trivial matter, including Death's sickles and Abaddon's locusts.
Valmanway in Dawn is the greatest weapon in the game, bar none. It may not have the highest attack power, and its the only weapon without a magical secondary attack, but its attack speed is ridiculous, and its the only weapon you can attack with and move at the same time.
It Was His Sled: The twist ending from the first game was a big deal. The second one starts right off with explaining what it was.
Magnificent Bitch: Celia Fortner. She's Affably Evil, plans her actions carefully, saves Dario at the last second because she's unwilling to let him get himself killed like Dmitrii was, and she learns from past mistakes.
Memetic Badass: Julius Belmont has become the Castlevania equivalent of Chuck Norris, being the one who killed Dracula permanently.
Scrappy Mechanic: The seal system in Dawn, which was obviously just shoehorned in to use the DS' touch screen capabilities (along with the Balore soul, which is only used in two rooms, including the room you find it in.)
Arguably the weapon crafting as well, which is the only way to get halfway decent weapons (almost crucial in Hard mode due to the increased durability of enemies), and these require certain souls and weapons which randomly drop. Worse still, some require boss souls, which there are only one of, and therefore if you decide to craft the weapon, you'll be unable to get 100% souls for that playthrough. Thank god for New Game+...
Of course, said boss soul weapons are the best in the game, so it's a hard decision to make.
Sweet Dreams Fuel: Very subtly, Mina, amongst the games that are so full of Nightmare Fuel. Just a sight of her is often enough to sooth both the player's and Soma's hearts full of fright. Her talisman, an extremely important item for Soma to confront the darkness in him, also serves as one.
That One Boss: Julius in Aria. He's fast, he's unpredictable, and he laughs at your Infinity+1 Sword due to his resistance to holy. You basically need an alternate weapon and Flash Step to beat him.
The battle can be trivialized if you know the right ways to do it. A max level Buer is easily capable of rendering the locusts useless (they die to 1 damage and they always collide with the fireballs) and the Frost Maiden soul can give you a temporary wall to use while you whale on him. Others still, like the Zombie, can be used as a temporary shield (which can be summoned multiple times). The problem in this is that while Soma has plenty of options, Julius and his crew do not. The most you have is Julius's whip which does... yeah... Not very helpful against the U shaped mobs and the death from above. Really the only way to stop it is to use the awesome power of the Cross Item crash, and that eats up around half your MP when you use it (thus it pretty much means you get 2 shots of it and it probably won't be close to killing Abaddon).
Or you can hold the whip and flip it around, or even use Alucard's special attack, which gives you invulnerability frames.
The whip only works on the direct attack. The Whip isn't fast enough to stop the death from above attacks. Julius isn't Nathan Graves. Alucard's invincible move also doesn't completely work out because he still has to go back to his original position and certain attacks last longer than his animation.
When playing as Soma, the easiest (if not argubly the best) way to win the battle is to just use a low-level Persephone soul attack. You'll literally suck up almost all of the locusts (while healing yourself in the process) and you can whale on the otherwise harmless boss. But those other methods sound good too.
Yoko's kicks from a safe spot work wonders for restoring your life.
Death's second part in Dawn. The first part isn't too bad, but the in second part, he Turns Red, and gets a lot of new and difficult attacks that can do major damage (especially the skulls). In hard mode, he also has ridiculous longevity aside from the strongest of the game's weapons, which requires a lot of money/soul grinding.
Death in Aria is no slouch, either. His first form isn't so bad if you have the Witch soul, letting you bounce his mini-scythes back at him For Massive Damage, but the second form becomes extremely tricky to avoid, especially when he starts rolling his scythe all over the walls and floor.
Can be a Wake-Up Call Boss in Julius mode as well: if you don't know how to use Julius's backstep or haven't really used it, that second form is going to be much harder.
Though Balore was a pretty huge pushover in Dawn, he's extremely tough in Aria, giving you little room to dodge his fists in the first half, and then constantly spamming sweeping eye lasers in the second half.
They Changed It, Now It Sucks: Dawn was the first game to showcase the new anime character art style replacing the classic gothic work of Ayami Kojima. It wasn't received well by Americans.