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.
Growing the Beard: When Bizenghast began, the first volume focused on the fact that Dinah was able to see and interact with the numerous ghosts that haunt her home. As the books progressed this aspect was near dropped entirely save for Dinah's interaction with the ghost of Addie Clark. On a wider scale, the series gradually moved away from the "save tortured souls" as the main plot and began to focus on Dinah's growth as a person and the overall dark history behind Bizenghast. The change is lampshaded by Edaniel in the sixth book:
Edaniel: It used to be so simple, huh? Solve riddle A, clear out vault B, learn valuable lesson C.
Harsher in Hindsight: In volume three Vincent is seeing wearing a shirt belonging to his father and saying it's too big. Dinah says he'll grow into it. Cue volume 4 and Vincent dying at the end.
Mary Sue: Subverted with Maphohetka. She has a Dark and Troubled Past from being burned to death by the intolerant townsfolk, she generates her own Bishie Sparkle, she's extremely magical, the Mausoleum guardians fawn over her, and everyone is more than happy to do as she wants. Turns out, the townsfolk chose correctly to Burn the Witch!, and she came back as a psychotic Charm Person who inflicts her will on others and who has additional arms and hands in place of her legs.
Nightmare Fuel: More than you can shake a stick at. Case in point, volume 8. Maphohetka's true form. It's not pretty.