In A Transatlantic Tunnel, Hurrah!, America lost the War of Independence and remained part of the British Empire. (Also, an earlier military defeat changed Spanish history enough that Christopher Columbus couldn't get funding, leaving American to be discovered by John Cabot in 1497.)
The Stars & Stripes series envisions the Trent incident from the American civil war blowing up into a full-scale war between the United States and the British Empire. A navigational error brings the Confederacy in on the Union side, ending the civil war, and resulting in the British getting their asses handed to them by Generals Lee, Grant, Sherman, and Stonewall Jackson.
Arbitrary Maximum Range: Starworld (part of the To the Stars trilogy) has the rebel admiral point out to the protagonist how energy weapons don't work due to the energy diffusion problem. Although missiles are being used by both sides, the rebels use linear accelerators firing unguided cannon balls to gain the decisive edge, then finish them off with a Flechette Storm of rocket-propelled bullets (fired from the standard infantry weapons of the time) which work well over infinite ranges due to the lack of air resistance.
BFG: One of the Israeli commandoes is firing a handheld .50 calibre recoilless machine gun during the attack on Spaceconcert in Starworld (part of the To the Stars trilogy).
Car Cushion: Played with in "Portrait of the Artist", in which a comic book artist, who has just lost his job to a machine, creates a suicide note in comic-book form that ends with a depiction of him jumping off his publisher's office building and landing on a car. After he commits suicide in the manner depicted, his ex-boss's only reaction is remark that he landed on the wrong car.
Child Soldiers: In "War With the Robots", the command staff are all teenagers as anyone older lacks the reflexes and flexibility of mind needed to fight the war. They retire after four or five years.
Generation Ships: In Captive Universe, a generation ship with a seamless environment is launched; by design the highly repressive, extremely stable Aztec cities onboard believe themselves to be in an inaccessible river valley. The ship tenders are if anything more rigid and religious: an extraordinary asceticism rules their lives and repairs are sacred rituals.
Government Drug Enforcement: In Homeworld, the upper-class protagonist is initially surprised at the idea that the proles might be rebellious, as the government lets them have all the drugs and booze they want.
Hideous Hangover Cure: The Drive-Right pill, which appears in more than one series, is a small round pill that will make you absolutely stone cold sober seconds after swallowing it... it's completely black except for a skull and crossbones on each side. Unfortunately it's rather unpleasant to take.
Humans Through Alien Eyes: In "The Streets of Ashkelon", a human missionary converts an alien culture to Christianity. The aliens then try to initiate the millennium of the missionary's message by crucifying him and waiting for him to rise on the third day.
Improbable Age: Justified in "War With the Robots"; the command staff are all teenagers as anyone older lacks the reflexes and flexibility of mind needed to plot strategy in the fast-paced war. They retire after four or five years.
Job-Stealing Robot: "Portrait of the Artist" is about a comic strip artist who loses his job to a machine.
Kinetic Weapons Are Just Better: Starworld (part of the To the Stars trilogy) has the rebel admiral explain to the protagonist why energy weapons don't work in the vast distances of space. Although missiles are being used by both sides, the rebels use linear accelerators firing unguided cannon balls to gain the decisive edge, then finish them off with a Flechette Storm of rocket-propelled bullets.
Mobile-Suit Human: The inverse (a human concealed inside a robotic alien suit) happens often in Harry Harrison sci-fi, such as Repairman and Star Smashers of the Galaxy Rangers.
Planet of Hats: In the To The Stars trilogy, EarthGov has not only terraformed Single Biome Planets, they've also created a unique culture for each in order to maximise their control. For instance the agricultural planet the protagonist has been exiled to in "Wheelworld" is populated entirely by peasants and mechanics, ruled by a group of autocratic Familys.
Robot War: In "War With the Robots", the human occupants of a command headquarters are forced out of their underground base by robot attack, leaving it to be manned by their own robots. On reaching the surface they find the enemy command staff living as farmers on the war-torn battlefield above — it turns out the robots on both sides find they can conduct the war more efficiently once humans are out of the way. The protagonist is deeply miffed.
Single-Biome Planet: Justified in the To The Stars trilogy, in which an imperialistic Earth has terraformed a number of planets (with a custom-made culture as well), each one dedicated to farming, production or mining of one particular resource. The idea being that none of them have the diverse resources needed to launch a revolt.
Space Opera: Parodied in Star Smashers of the Galaxy Rangers.
Unobtainium: The Golden Age SF spoof novel Star Smashers of the Galaxy Rangers features Cheddite (a fuel created from cheese). In another scene the heroes' 747 jet is turned into a spacecraft by means of windows armored with armolite, vacuum insulation with insulite, fuel tanks filled with combustite, guns firing pellets of destructite, batteries replaced with capacitite and a space-warp drive powered by warpite.
The War of Earthly Aggression: The To the Stars trilogy has a Big Brother-like Earth lording it over interstellar colonies set up to be totally dependent upon each other. Since each colony requires numerous goods (which they are never allowed to stockpile) each made only on one of the other colonies, it would be impossible for a revolt to succeed unless every colony did so at once. Which they do. (It's not not strictly Earth-vs-everyone-else, though. On Earth itself there are several rogue states that cling to old ideals, such as democracy, the strongest of them being Israel. The last novel makes it clear that a revolution can only succeed with a simultaneous assault on the surface and space.)
What Is This Thing You Call Love?: Satirized in "The Robot Who Wanted To Know". Sophisticated robot librarians designed to think independently often focus on a particular area of interest; Filer 13B-445K's interest is human concepts of love and romance. After reading up on it he wants to experience it personally and goes to some lengths to disguise himself as an attractive man for a costume ball. Naturally the busty heroine ends up falling for him and is outraged to discover his mechanical identity. He responds by nosediving into a paradox spiral and self-destructing. Workers examining the wreck later find a malfunction in the central pump and joke that "you could almost say he died of a broken heart".
You Are in Command Now: In Spaceship Medic, the eponymous medic ends up in charge after all the rest of the ship's officers are wiped out by a meteorite hitting the bridge (then has some adventures, saves the day, and goes back to being a doctor).