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.
Ensemble Darkhorse: Grace is arguably more popular than the titular character. The character has her own Wikipedia page while Gabriel doesn't, is Promoted to Playable in the sequels, and people generally find her parts of GK 2 and 3 to be more interesting than Gabriel's.
Foe Yay: Gabriel and Von Glower — very intentionally too.
Harsher in Hindsight: In Sins of the Fathers, it's possible to have a conversation with the gravedigger where he outlines in graphic detail what would happen to the bodies buried in New Orleans if there was a flood, twelve years before Hurricane Katrina.
Ho Yay: plenty in The Beast Within. Specifically, von Glower's past lovers include Ludwig and von Zell, and he's clearly interested in Gabriel as well.
Nightmare Fuel: Wolfgang tears his own heart out so Gabriel can get the Schattenjäger amulet.
Ooh, Me Accent's Slipping: Curry actually does a fairly convincing Southern-American accent about 99% of the time, but every once in awhile you'll hear a critical fail that wasn't redone.
Narm Charm: This actually is probably the only "bad" voice acting in the game, sans the Narrator. When it fails, it's an Epic Fail but that's what adds to the charm.
Curry repeatedly pronounces "New Orleans" as "New Orleens", which is obviously something a true New Orleanian (or someone who's watched Treme) would never do.
Paranoia Fuel: In Sins of the Fathers once you find out about the Rada Codes. Then you notice the drummers stationed all over the city. You know, the ones you've probably been ignoring because they just sit there all day and drum. They're not only watching you, they're broadcasting your moves to the creepy voodoo cartel that's already left a number of messy corpses all over the city. And of course, the creepy looking guy who just stands outside the shop for two days and stares in the window.
Special Effect Failure: The Beast Within, when it comes to the Werewolves. They are completely computer generated and looked awful(but not in the intended way) when the game was released. The fact that the Normal wolves are depicted via video footage of actual wolves makes the werewolves look even worse by comparison. The damage is limited by the fact the wolf-form werewolves get relatively little screen time until the end.