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.
Anti-Climax Boss: Despite years of build up, staying off screen and being the cornerstone by which much of Hellboy's mythology is based, Satan goes down incredibly easily.
Complete Monster: In the dark world of Hellboy, Nimue, the Queen of Blood, reigns supreme as the greatest monster. Once a mighty sorceress who betrayed the great Wizard Merlin and imprisoned him to live eternally in his own grave, Nimue was imprisoned for 1,500 years by the witches of Britain. Upon her revival to lead the Wild Hunt, Nimue destroys the other witches of Britain and sets about marshaling an army of the Fae. When one ambassador comes to her to say his king will stand with her, Nimue forces him to murder his king, come out his heart and forge her a new crown with it. Nimue launches vicious purges to ensure there is no threat to her rule, intending to exterminate humanity to the last man. When defeated and being dragged to hell by the witches she'd murdered, Nimue spitefully tears out Hellboy's heart to ensure she won't face damnation alone.
Engaging Chevrons: Parodied in the first movie, a guy with a high-tech flamethrower backpack goes through a long, drawn-out start-up sequence, only to be eaten by the thing he's trying to kill before he can use it.
Even Better Sequel: Despite diverging even more from the source material, The Golden Army is liked just as much as, if not more so, than the first film.
Although it's notable that Mignola eventually eschewed the pulpier villains and Nazis of the earlier stories for a similar path through folklore and mythology, while the BPRD series handled threats that had been notched up early in Hellboy's run.
Fridge Brilliance: Nuada's fighting style is mostly based on speed and agility, doing all kinds of elaborate maneuvers to land hits on his opponent without being hit back. And he shares a magical link with his twin sister that means that everything that physically happens to him also happens to her and vice/versa. It stands to reason that he developed his fighting style in order to keep his sister from getting hurt along with him, supported by the first fight scene where he gets hit in the face and then turns to Nuala, clearly horrified as they both start bleeding.
Hellboy's Kind Hearted Cat Lover traits make sense due to his hellish origins. Cats are believed to be the gatekeepers of the underworld.
Ho Yay: It says something about the writers (or maybe the actors) that one of the most romantic scene in the either of the films is two dudes getting drunk and singing "Can't Smile Without You" to each other.
Moral Event Horizon: Prince Nuada starts off by unleashing a swarm of ravenous tooth fairies in a room full of people, the tooth fairies ate every last bit of everyone in that room till there was nothing left of them. There is absolutely no purpose to this either, as he already has what he came for. He does it just because they're humans.
The Problem with Licensed Games: Both Hellboy games had outdated graphics and repetitive gameplay, and the first one was full of bugs. Although The Science of Evil was supervised by Mignola and Del Toro, it didn't make up for the programming shortcomings.