The story of what normal, decent people are capable of doing to an old lady when they suspect her of being a witch (kill her cat, burn her cottage, leave her to starve). Tiffany's reaction, even years later, is heartbreaking. Pratchett is a master at describing the banality of evil.
The Drome in the Jolly Sailor dream, unexpectedly for a creature that had before that been depicted solely as predatory Paranoia Fuel.
"It thinks it's home, Tiffany thought. I've given it a dream it likes."
When Tiffany defeats the Fairy Queen:
"I never cried for Granny because there was no need to," she said. "She has never left me!"
There's also the part where Granny Aching appears to Tiffany. Before she died, Tiffany had given her a little china statue of a shepherdess, but worried that it might be taken as an insult because her grandmother looked nothing like it. When she appears, Granny Aching is wearing the statue's outfit - a sign of how much it meant to her anyway.