Particularly how they were contagious. Polly didn't have any idea what helicopters were, hadn't even heard the term ... but she could almost hear the rotors and nearly saw them coming into the "LZ"....
The entire sequence where the girls dressed as boys dress up as girls to sneak into the enemy's fortress. And nearly don't pass. In particular:
"Did you bring a weapon, Tonk — Magda?" "No, Polly." "No item of any sort with a certain weaponlike quality?" "No, Polly." "Anything, perhaps, with an edge?" "Oh, you mean this?" "Yes, Magda." "Well, a woman can carry a knife, can't she?" "It's a sabre, Magda. You're trying to hide it, but it's a sabre." "But I'm only using it like a knife, Polly." "It's three feet long, Magda." "Size isn't important, Polly." "Nobody believes that."
When the soldiers dress as washerwomen in order to sneak into the hold, the only one who convinces the guards that he's a female is Blouse, while the others are declared to be "obviously male". The hilarious part is that Blouse is the only actual male in the group.
And how Shufti proves her (and presumably everyone else's) gender, and Polly's admission later that "Things got a bit hairy for a moment..."
The description of how knowing too much about birds rather spoils one's enjoyment of birdsong: "It's hard to be an ornithologist and walk through a wood when all around you the world is shouting 'Bugger off, this is my bush! Arrg, the nest thief! Have sex with me, I can make my chest big and red!'
Nuggan's Abominations. And Vimes agreeing with him about the accordion players.
When the squad encounters a group of enemy soldiers, beats their asses, steals their clothes (and their boots), chains them up and drops the key into the privy - made doubly karmic and hilarious by the leader, Captain Horentz, turning out to be Prince Heinrich, ruler of the opposing country (and a total asshole). It's made even more brilliant when William de Worde turns up with Otto von Chriek and, to quote the article they wrote,
Vimes: I bet de Worde would have let him out if he'd been nice and polite. That'll teach him to meddle with the awesome power of a free and fair press.
And that's how much Vimes dislikes him. He tends to think of William as a necessary complication of his job at the best of times; him agreeing wholeheartedly with something de Worde did?
A Zlobenian captain irritates Polly by asking if she's one of those... and did they really... in shackles and no clothes... and they're just women? Then he leans forward:
"Dan gug show. Ell done. Agout time soes arragunk arsetards ere aken own a eg! Commander Vimes it is, then. Follow me, miss."
Jackrum, Polly and Shufti steal women's clothes from a brothel, requiring them to very temporarily go undercover (as johns, not prostitutes).
The squad disguising themselves as trees, getting ordered to 'spruce up!'
The Running Gag of Polly (who's never actually shaved anyone in her life) having to shave the Lieutenant, only to be saved at the last possible moment.
In an instant of Black Comedy, Polly reminding Wazzer that while praying is a good thing, there's a time and place for it, and standing in another man's grave really isn't that time or place.
Late in the book, we find that giving someone the thumbs-up and winking at them means either 'Jolly good' (in Ankh-Morpork) or 'I hope your donkey explodes' (in Klatch). Shufti then asks why the person making that gesture would have any reason to applaud them, or hate their donkey so much.
The brief insight into the mental progress of pigeons, which is revealed to be simply: "000000000!"