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.
Foe Yay: A metric ton of it, courtesy of Isaac to Hector.
Also Isaac to Trevor - kissing his hair after impaling him from behind.
Fridge Brilliance: Julia, who is Isaac's sister, is a witch who escaped the witch hunts and inexplicably looks identical to Hector's wife Rosaly, who was killed in said witch hunts thanks to Isaac. Could it be that Isaac framed Rosaly both to piss off Hector and to protect his sister...?
Game Breaker: Corpsey's Bone Storm. It deals a lot of damage to the enemies, makes Corpsey invincible while using it, can stunlock some enemies, and is cheap to use too.
Goddamn Bats: There are enemies that qualify as this trope, but oddly enough, there aren't any actual bat enemies in Curse of Darkness at all, unlike the rest of the series.
Ho Yay: Between Isaac and Hector, Isaac and Trevor... okay, between Isaac and everyone but Julia (who is his sister).
Narm: The dialogue can get like this, as it tries to sound Shakespearean ("'Tis done" etc.).
Nightmare Fuel: The dungeons of the Garibaldi Temple, especially outside Legion's room. And Legion itself.
That One Boss: Every boss starting with the first encounter with Trevor Belmont is That One Boss. Especially Dullahan, what with his shockwave attack. And the boss before him? Trevor, again. He's even worse the second time.
That One Level: Dracula's Castle is easily among the worst levels in the game. It's not particularly hard, per se, but going through the same few rooms OVER and OVER again gets to be a hell of a drag.
That One Sidequest: Some of the game's better and... wackier weapons require hard to find items, some of which are boss-only steals. Stealing items requires the enemies to be in a particular frame of motion. Oh, and did you happen to miss a key item from a boss? No worries, you can fight them again! Provided you boss rush through all the bosses in the game that is. Not too bad, until you remember that every boss past the first two are That One Boss. One of them (Death) actually requires you to raise up a fairy with the ability Invincibility Jar to get to him when he's vulnerable to stealing and time it so that it runs out right when his Sphere of Destruction attack ends (since you can't steal while invincible), or learn how to perfect block all its hits.
There's a slightly easier way to do the above by turning down the music volume so that you can hear when the attack is about to end and switching IDs causes the invincibility effect to end instantly, eliminating the exact timing element to a large degree.