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: Draco Black. Even the author has commented on the unexpected popularity of the character. News that the character was apparently going to die in the not-too-distant future caused much outcry among the fandom.
Nor was she the only one to mention that learning about Portkeys could come in handy.
Or a hidden swimming pool.
Helga Hufflepuff's nickname being "Gaga" might qualify.
Mary Sue: The titular character is a Werewolf Tamer and can speak to Remus via telepathy.
Though she's less annoying than most examples given how she's not a Flat Character.
Nightmare Fuel: The methods the purebloods use to perpetuate the family line. A pureblood daughter is either given a list or has the decision on whom to marry made for her; she is then dosed up with a fertility potion and locked in a room with her 'husband.' Oh, and her kind parents make sure to cast silencing charms on the room so they can claim they never heard the screaming.
Schedule Slip: it took 5 years for Facing Danger (book 4) to be completed. Surpassing Danger (book 5) is thus far averaging slightly less than one chapter per month.