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: Wyrd Sisters
Complete Monster: Lady Felmet is modeled after Lady Macbeth, only multitudes more sadistic and cruel. The Duchess participated in the overthrow and murder of King Verence I for her husband Leonald to gain the throne for her own benefit and power. Once she inherits her position, she encourages a brutal suppression of peasants and witches alike while using the Fool to spread a propaganda campaign. Revealing herself as a sadist who loves Cold-Blooded Torture, Lady Felmet personally engages in inventive torture with captives. When Granny Weatherwax removes the mental walls she put up and shows her all the evil in her own mind, she shrugs it off. Unlike most Discworld villains, the Duchess is fully aware how evil she is. She just doesn't care.
Fridge Logic: Some of the ramifactions of the witches moving Lancre in time. The people don't generally seem too dismayed, but fans have wondered about Esk, the Lancre-born female wizard from Equal Rites. This may possibly be alluded to in I Shall Wear Midnight - but possibly not.
It also crops up in later books - shouldn't Granny Weatherwax now be another fifteen years younger than her childhood acquaintances like her sister Lily (Witches Abroad) or Mustrum Ridcully (Lords and Ladies), not to mention when she goes to visit Mrs Palm again in Maskerade (having previously met her in Equal Rites)? At this point one really just has to resort to the MST3K Mantra.
Mustrum Ridcully commented that she hadn't changed a bit, so maybe appearing 15 years younger than expected helped.
Lily was already older than Esme. Her relatively-youthful appearance is due to all the ethical tensions in Esme's life, that Lily never allowed to bother her. Lily's abuse of mirror-magic may also be somewhat to blame for her unchanging looks.
Mrs. Palm has been with the Seamstresses since before Vimes joined the force in Winder-era Night Watch. By Guards! Guards! she's old enough for Carrot to assume she's the mother of the other prostitutes, and by Maskerade she's thinking about retirement.
This is actually one of the reasons Terry eventually wrote Thief of Time, to explain away the wibbly-wobbly, timey-wimey nature of Discworld chronology. It all makes a twisted sort of sense that the progression of time is inconsistent between places.
Discworld also tends to take physics and exaggerate somewhat (such as the oft-mentioned relative slowness of light); time does move at somewhat different rates at different elevations. And Lancre is much higher up than Ankh Morpork.
Fridge Brilliance: It's fairly obvious that the plot is based on that of Macbeth, but only those who know the background of the play (or read about it later) know that Macbeth is also like that: in Real Life, the historical Macbeth is usually considered to be the good guy and Shakespeare wrote the play in order to appeal to King James VI and I, who was the descendant of King Duncan.
Genius Bonus: The Felmets' plan (based on the Fool's ideas) is to have Hwel write a play that presents them as the good guys and King Verence and the witches as villains, altering public perceptions. This is exactly what the original play Macbeth was, with Shakespeare whitewashing his patron King James I and VI's ancestors and painting Macbeth as a blackguard, when the historical record suggests he was seen as rather a decent king.
One-Scene Wonder: In the Animated Adaptation, the demon the witches sum up becomes this in a way he wasn't in the book — partly this is because he's the best-looking, best-animated character in the miniseries, but a lot of it is also due to the added joke of his voice suddenly goes from Evil Sounds Deep to high-pitched Upper-Class Twit when Granny tells him to stop mucking about.