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: Castlevania: Circle of the Moon
Anticlimax Boss: Death, who is pathetically easy compared to some of the other bosses. Especially egregious since you fight him after the Zombie Dragons.
Game Breaker: Crosses. They deal about as much damage as your whip, but can actually hit at least three times if you measure the distance properly, since they stay in the air for a while once they reach their maximum range. They're very hard to find, though.
The Uranus DSS card, too, which can summon creatures to deal massive damage to everything on the screen, and one of them is the only healing spell in the entire game. (Other than Jupiter+Mandragora, although that only works via slow regeneration) You'll only find it late in the game, though, and even then, only if you have a guide.
Thief Mode, which turns the frustratingly low Random Drops back into your favor pretty fast. Although it bumps down your other stats, making the beginning of the game harder.
Magician Mode, where your intelligence stat is so high that you fully restore mp in a matter of seconds. By level 33, your mp recovery rate is so high that even your healing magic doesn't drain your mp at all. There is also the Pluto+Unicorn combo, which grants you full invulnerability. In other modes, you only get the Unicorn card after the Battle Area, so it doesn't really get used much, however in Magician mode, you have access to it from the very start of the game.
Fighter Mode puts your attack strength and defenses through the roof at the cost of not being able to use magic. But who needs magic when your enemies are about as effective at hitting you as a blind man is at driving and you can kill any of them in 3 lashes of the whip maximum?
Goddamned Bats: Goddamned Bloody Swords, small targets which alternate between hovering in place and lunging at Nathan full-stop.
The marionettes in the same area with similar (though slower) movement traits, who can also curse you (no attack). And when you have 3 of each tailing you thirsting for blood...
Good Bad Bugs: Through a glitch, you can use any DSS Card combination after you get your first one. After casting the spell but before the effect kicks in, change the DSS combo to whatever and you'll be able to use that combo. You can also change your card combination to something that boosts your strength while in the middle of a summon spell to greatly boost its' damage.
The Bonus Dungeon is supposed to prevent you from casting magic. They implemented this with an insane Mana Drain effect, but you can still chug a potion or five and quickly cast something before you run out of MP.
Though you are prevented from attacking enemies while cursed, you may still damage them by sliding into them. This doesn't completely alleviate the problems with the areas with curse-inducing enemies, but it does help some.
Nightmare Fuel: Hugh, especially during his fight against you. You can't imagine when you are being mind-controlled by Dracula and not knowing what the hell you are doing until it is too late.
Scrappy Mechanic: There is no "suspend" feature. Coupled with the lack of save point density, playing this game on short breaks may not be such a good idea. Konami would learn their lesson come next game, however.
That One Attack: Dracula's One-Winged Angel form will start spamming a charge attack at low HP which takes up half the screen, will wipe out almost all your health and is virtually impossible to antipate the first time (though his glowing might be an invitation to get the hell away, you need to go up specifically). At low enough health he will use nothing else. And don't get started on his Goddamned Bats which makes it nigh impossible for you to get close to and whip him...
That One Boss: The minotaur room in the arena. Seven of them, each with 700 hp and a ridiculously powerful attack that reaches halfway through the screen, both horizontally and vertically . Halfway into the dungeon, far away from the save point, no magic, and nowhere to run.
Zombie. Dragons. You don't have a lot of room, they like to swing wildly and drop random debris from the ceiling, and when you kill one, the other literally devours the other to gain 1000 hp. They're also above you, making attacking them awkward. The Ax is your friend (for once).
And the goddamn Chapel with the marionettes EVERYWHERE which curse you, making you unable to attack, and worse still, bloody swords which move in an erratic pattern and are hard to hit even when you can attack. Both enemies are spammed like hell.
There's also the Gallery; a segment that's filled with enemies that can poison you with little room to maneuver past them all.
While the Battle Arena is hard through out, two rooms stand out as difficult even by the Arena's standards - one is wall to wall Minotaurs, and the other is filled with Poison Armors and Bloody Swords.