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: I Shall Wear Midnight
Anvilicious: Any witch hunt is an allegory for any other, but Pratchett included an innocent victim who had a book of Klatchian poetry to make sure we Get It.
Complete Monster: The trial of one particular convict apparently dragged on for a while, becase the jury kept throwing up. Also the Cunning Man, who in possessing the aforementioned man forces him to kill his pet song bird. And somehow, this is the worst crime of all.
See Moral Event Horizon below. The song birds kept by the condemned criminals are symbols of hope and redemption, so killing one is this. (This being Discworld, where all symbols are real...)
And the Cunning Man is an embodiment of lack of forgiveness, and thrives when those he manipulates refuse to acknowledge that witches are human beings. Giving canaries to prisoners is society's acknowledgement that the condemned men aren't beyond human feelings, and the prisoners prove that's true by caring for them. The Cunning Man would make the convict kill his bird!
The Cunning Man crosses it in the readers' eyes (in case you didn't realize how bad he was) with the canary. More importantly, that's when he crosses it for Tiffany. It's the reason she accepts that he's beyond redemption, and no longer human enough for her to hesitate about vanquishing.
Nightmare Fuel: The Cunning Man. He is the personification of blind hate and lynch mobs.
Relationship Sue: You have to wonder a bit about Preston, especially the way he shares Tiffany's obsession with words.
They Wasted a Perfectly Good Plot: You would think the existence of a human girl with kelda abilities would be a big deal, but it has absolutely zero effect on the plot.
Kelda abilities are implied to be another form of witchcraft, closer to Nanny Ogg's methods than the Granny Weatherwax form Tiffany uses. Combined with Letitia's bookish magic (more like Magrat's), it serves to make the Chalk coven a mirror of the original Lancre coven.
It's also probable that Pratchett intended Amber's abilities as plot fodder for a later Tiffany Aching book.