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.
YMMV: Magic Kingdom of Landover
Canon Sue: Arguably, Mistaya. Aside from being the princess, doted on by her parents, the instrument of her grandfather burying the hatchet with Ben, tutored (and adored) by Questor and Abernathy, and loved and fiercely defended by Strabo, she is a child of Landover, Earth, and the fairy mists and thus has incredible magical power which manifests from birth. She ages rapidly, as well as having a far more adult mind before her time, and her magic which has already proven to be on a par with Nightshade's is only getting stronger with age. The only things that keep her from being a true Mary Sue are the fact there are others remaining with equally powerful magic (Ben, Nightshade, Strabo) to keep the narrative balanced, that as shown by Princess even she doesn't know everything and often needs help, and the fact that in the end she still manages to be rather likable.
Moral Event Horizon: Using the Darkling to compel Willow's mother to dance for him came very close to this for the River Master. Luckily he realized in time and let her go.
Nightmare Fuel: An Ardsheal is an ungodly-fast, super-strong, nigh-indestructable humanoid elemental that is virtually invisible when it wants to be. In Witches Brew, the River Master summons one as a bodyguard for Willow and Ben. Then when Nightshade gets a hold of it we find out exactly why Willow is terrified of it.
True Neutral: At first the fairies seem to hew to this trope, but eventually it is revealed that they will take sides, if it means preventing the destruction of Landover or maintaining the Balance Between Good and Evil (or Law and Chaos). And every time they do choose to act, it is always the key to salvation: in book 1 their gift of Io Dust is what allows Ben to get rid of Nightshade and stop Strabo's depredations; in book 2 Dirk's assistance to Ben and the gradual alteration of Willow's dreams allows them to stop Meeks and save the unicorns; in book 4 it is the fairies allowing Willow and Ben to encounter each other in the mists, circumventing the Tangle Box, that helps him recover his memories of his true self (and them sending Dirk to Willow allows her to gather all the soils she needs and give birth to Mistaya); and in the most recent book they send Dirk again, this time to Mistaya to help her save Libiris (and Landover) from Crabbit and the demons.