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.
Magnificent Bastard: Tertius Fume. Between dragging Alther into the Darke Halls when he's Banished and sending Septimus on the Queste with a rigged Draw, he's definitively a magnificent antagonist, especially compared to his rather dull predecessors.
Nightmare Retardant: The Things' faces, as shown in Darke, look almost weird, rather than scary.
Shipping. Many: Septimus/Jenna, Marcia/Milo, Marcia/Marcellus... the list goes on and on.
The Scrappy: Milo Banda is likely the only character in the series who is disliked by some fans due to him being constantly away on travels while his daughter Jenna and the Heap family are in trouble and being generally embarrassing for Jenna.
Uncanny Valley: Septimus's illustration in Darke can be a little disconcerting, what with the absent-minded and distanced look.