The best part being that in the epilogue, Sam is surprised to find that Jane's novel Pride and Extreme Prejudice is now dedicated to him.
Young Sam's continuing obsession with poo.
"I've got a laboratory in the lavatory."
Young Sam learns to milk a goat.
"Dad! I know how to milk a goat, Dad! You have to pull its tits, Dad, they're all wiggly!"
The titles of the children's books.
The bathroom in Ramkin Hall, where the decorator had gone rather overboard with Faun motifs. As Sybil said, use of the bathroom when she was a child rather ensured that she wouldn't be shocked in later life.
The explanation of the rules of crocket, which takes so long that (metaphorically presumably, but with Pratchett you never know), the universe ages and ends, a new universe is born, life evolves, a new Sam Vimes exists, and he's there just in time to hear the end.
Early on, the explanation of why Vimes never argued with Sybil:
...because even when he thought he had won, it would later turn out, due to some magic unavailable to husbands, that he had been misinformed.
Vimes uneasily reflecting that he had always thought of authors as people who just lounged around in their dressing gowns all day, drinking champagne, and a footnote asserts that this is, of course, completely true.
And in at least one language, that footnote has a translator's footnote stating that this must be true, since Mr. Pratchett knows what he's talking about.