An explanation: Sam Vimes has been fighting the unstoppable influence of the Summoning Dark, a 10,000 year old quasi-demonic thing of pure vengeance, for the entire book. It's a substition that normally drives a mind into blind, bloody rage against the enemies of whoever invoked it, and he's been holding it at bay with his belief in law and justice. What finally motivates him to give in is that it's six o'clock, and at six o'clock every day he's at home reading Where's My Cow to his son. He then turns into a striding, screaming, vengeful maniac who fights his way alone through a crowd of heavily armed dwarfsnote Heavily armed even for dwarfs. Flamethrowers and everything., until he reaches the Summoning Dark's true targets and... stops. Because Sam Vimes doesn't kill helpless cowering old men.
Vetinari: Given, then, a contest between an invisible and very powerful quasi-demonic thing of pure vengeance on the one hand, and the commander on the other, where would you wager, say... one dollar? Drumknott: I wouldn't, sir. That looks like one that would go to the judges.
Vetinari: Sam Vimes once arrested me for treason... And Sam Vimes once arrested a dragon. Sam Vimes stopped a war between nations by arresting two high commands. He's an arresting fellow, Sam Vimes. Sam Vimes killed a werewolf with his bare hands, and carries law with him, like a lamp... Watchmen across half the continent will say that Sam Vimes is as straight as an arrow, can't be corrupted, won't be turned, never took a bribe.
This includes a bit of foreshadowing, as the metaphorical lamp is used by Vimes' inner watchman, his internal spirit of justice that watches his every word, action, and thought for corruption, to drive out a demon of pure dwarfish vengeance older than time itself for a CMOA that takes place entirely in Vimes' head:
The Watchman shook his head. "I don't think you understand," he said calmly, "I'm not here to keep the darkness out. I'm here to keep it in. Call me ... the Guarding Dark. Imagine how powerful I must be.
Made better in the audiobook where The Watchman and Vimes have the same voice.
Detritus' crusade against drugs:
Vimes (about some drug makers recently arrested): They didn't know what hit them, eh? Detritus (a bit indignant): Oh no, sir. I made sure they knew I hit 'em.
Detritus' story in general: he went from less-than-a-nobody to a respected sergent and key element of the Troll/Dwarf truce, and from a very secondary character to the fourth main protagonist of the City Watch arc (the three others being, of course, Vimes, Carrot and Angua).
Detritus is also capable of preventing Vimes from murdering a room full of men who like to do nasty things to little girls. Anyone who knows Vimes will be impressed at that, even if Detritus is a troll.
Nearly any time Detritus fires The Piecemaker. It is a ballista converted into a shotgun crossbow with a sheaf of bolts. In practice, it ends up firing an expanding cloud of fire and splinters, due to the massive forces involved.
"When Mister Safety Catch Are Not On, Mister Crossbow Are Not Your Friend."
A.E. Pessimal, a bureaucrat assigned to the Ankh-Morpork City Watch by Vetinari, attacked a rampaging, crazed troll with his teeth, an act of courage so outrageous and unexpected, the Patrician literally didn't believe it when he was told.
Discworld trolls, by the way, being solid living stone, with teeth of diamond.
Pessimal has one a bit earlier:
COME ON, THEN, IF YOU THINK YOU'RE HARD ENOUGH!
Vimes's butler Willikins has these moments as well. He once bit someone's nose off, (in Jingo) and this time he defeats a couple of dwarfs who had attempted a sneak attack upon the house.
During the same attack, Sybil and her dragons get their CMOA. The dwarfs attack her with a flamethrower, but since she's wearing an outfit designed to protect her from her dragons (who have a habit of belching flame and exploding with little provocation), she shrugs the attack off. The dragons, however, see the flamethrower as a challenge... and twenty-six of them simultaneously incinerate the poor bugger (leaving a white-hot glowing pair of iron boots and a scorch mark in the shape of the attacker), with Sybil simply saying, "Good boys."
And her Moment continues when, rather than panicking or getting shaky, she takes charge of the situation (from Vimes, no less!), instructing him to arm himself with one of her dragons as they prepare to carry their son to safety in the Watch house.
Sybil: Coal him up.
Angua in the end, when she convinced the demon-possessed Vimes that now would be a great opportunity to not be moving. Let it be noted that the reason she had to convince him to stop moving was not because he was about to kill the dwarfs in front of him but because he was going to start hurting himself from the strain of not doing that. So she knocked him down.
A minor one for Nobby, whose patented Nobbs Sidle proves stealthy enough to slip past the Low King's best soldiers and pocket the very artifact they're frantically combing a cavern for.
Forbidden to reveal what was happening to anyone, and terrified of his superiors and the Summoning Dark, Helmclever still managed to ensure that Vimes would enter the mine by making it clear that the deep-downers were trying to prevent the wider dwarf community from speaking to the Watch, provoking the Commander's everlasting ire. A good Thud player to the end.
Vimes discovering the power of the mark the Summoning Dark left on his forearm when the Low King asks him to hand over the Codex.
Vimes: Take it.
Rhys: I ask you to give it to me, Commander.
This scene with Death, which got partly adapted into the Colour Of Magic film:
Death:You are having a near-death experience. Which inescapably means that I must undergo a near-Vimes experience. Don't mind me. Carry on with whatever you were doing. I have a book.
Bashfullson is crowningly awesome as an embodiment of how to be both Morporkian and Dwarfish at the same time.
It has been repeatedly made clear that a dwarf without an axe is not a dwarf. Vimes, after a dwarf attack on his home and with every reason to hate dwarfs and a 10000 year old quasi-demonic thing of pure vengeance riding in his head orders a group of dwarfs to disarm but still concedes one axe each. Grag Bashfullsson doesn't carry one. At the end of the book there is a "discussion" between Grags about how to be a dwarf.
Grag Bashfullsson: No! Sire, please! This is an argument between Grags!
Grag Ardent: Why do you carry no axe?
Bashfullsson: I need no axe to be a dwarf. Nor do I need to hate trolls. What kind of creature defines itself by hatred?
Ardent: You strike at the very root of us! At the root!
Bashfullsson: Then strike back. [holds out empty hands] And put your sword away, Commander Vimes. This is dwarf business. Ardent? I'm still standing. What do you believe in? Ha'ak! Ga strak ja'ada! Ardent attacks, Bashfullsson defends. A shocked Ardent is silenced, and falls.
Bashfullsson: It is like using an axe, but without the axe.
He effortlessly translates what the cube says, and has to shout "I WHO SPEAK TO YOU NOW AM B'HRIAN BLOOD-AXE, BY RIGHT OF THE SCONE THE TRUE KING OF THE DWARFS!"
Bashfulsson talked Vimes into letting him supervise his interrogation of Helmclever. Given that Vimes was in a white-hot fury and clearly ready to take his anger out on just about any dwarf who looked at him wrong, and that Bashfulsson had good reason to believe that he was being influenced by the demonic vengeance creature that had infected him without his knowledge, getting him to see reason was not only brave but also pretty damn impressive. He actually left Vimes speechless.
Vimes: I'm not in the habit of beating up prisoners, if that's what you're implying.
Bashfulsson: Then I assume that you don't want to start tonight.