Warhammer 40,000 is one of the most Trope Overdosed media pages we have and is currently the highest Wicked media page. However most of its tropes can be sourced back to the use of a few parent tropes and its focus as a tabletop game.
The Rule of CoolThe game of Warhammer and its futuristic offspring both involve pitting armies against each other, represented by miniatures sold by the intellectual property owners, Games Workshop. Therefore GW has a vested interest in getting lots of different people to play lots of different armies. If one person wants to play the heroic Space Marines, they will sell them Space Marines, if another person hates the typical Sci-Fi Space Marine template perhaps they would like to spend $49.95 on a box of wild, violent, asexual Orks (green paint not included). However, each race needs background descriptions on which they are sold to each personality, so that one race doesn't become woefully less popular than the other and become unprofitable. They all need their cool moments and those cool moments need to be readily demonstrated to each potential player on a first glance. Warhammer 40,000 ends up dominated by Rule of Cool and all its subtropes. Any particular variation is included not just because somebody at HQ has a chainsaw fetish (though they probably do), but because somebody somewhere will find it cool. Chainsaw Good, Abnormal Ammo, BFGs and Guns Akimbo? All there so that somebody can describe with glee the walking tank that shoots chainsaws made of human bones out of both its kneecaps. All of these things are pushed Serial Escalation so that they can then become more memorable and iconic. Once you have a few uses of Beyond the Impossible, people will get the message that this is a game that doesn't worry about pesky little things like the laws of physics. This makes the Rule of Cool easier to implement without complaints.
GRIMDARKThe game itself needs a draw that distinguishes it from other sci-fi settings. The tagline is "In the grim darkness of the far future, there is only war" and boy, do they mean it. The Forty-First millennium is a Crapsack Universe with Black And Black Morality. If you want everybody to be at war with each other and for each player to be equally right, then the easiest thing to do is make them equally wrong. Some people find the GRIMDARK! setting to be restrictive and a tad childish. Any theoretical question an interested player has is to be solved using chainsaws and the blood of small children. Grimdark makes for easier writing. If the Rule of Cool can't help you decide who wins in a fight, the Rule of Misery will. Dark Angles Chapter Master versus an avatar of the god of murder? Can't decide what happens? They all get eaten by Tyranids.
A decent use of an entire galaxyOne thing Warhammer 40,000 does better than some other sci-fi settings is getting the idea of a big galaxy with lots of things going on across it. It lets the writers say "Sure Why Not", since any one depiction of an army's behaviour or organization can be represented on some planet somewhere. Players get to make up their own Space Marine chapters or Eldar craftworlds. They can have nice, noble marines inspired by Aztec rainbow warriors or crusading religious zealots thirsty for the blood of heretics. The Imperial Guard can be the worthless Cannon Fodder or among the greatest Badasses n the setting. A planet can be a Coruscant-like hiveworld or Planet of Hats or it can have its own complex class system. Compare 40K to Star Trek where we only ever get to see one corner of the galaxy which is mostly populated by a mono-cultural Federation. Even the bold new frontier is full of humanoid aliens or glowy lights. The problem is that Star Trek is a weekly show where each new race has to be introduced and explained and realised by the make up and prompts department. In the Warhammer army books, however, all you need to do is doodle some bizarre looking alien in the margins and the fans can go off and imagine what they are about themselves. The game has been helped by the foreshadowing that has occurred in those margins. Images of the Kroot and the Demiurge have shown up well before they became playable armies. Also, references on maps to the Tannhäuser Gate. Things like the Kroot could have been people looking at the pictures then saying "Hey that would be cool"
No Need For Originality / A Mega Crossover Fan Fic settingYeah, you heard that right. Warhammer and 40K aren't that original. Lots of things have been taken from Michael Moorcock, Alien, Starship Troopers and even the Terminator movies. Even 40K was originally just Warhammer IN SPACE!. This isn't necessarily a bad thing, though. Since the meat and bones of the fun is meant to be the tabletop wargames, the background can be a chance to engage in some geekery indulgence and Shout-Out references. You'll see things mixed together you normally wouldn't, and even get a chance to act them out in the game. Then things from different sci-fi settings can (sort of) be pitched against each other. Want to know who would win in a fight between the Alien Queen and Cthulhu? Put a Tyranid Hive Tyrant against a Lord of Change. This then allows one to bring in all the tropes of the original works as well as putting in the Applied Phlebotinum tropes family so that a patchwork job of the imagery of different technological levels can exist alongside each other. Tanks that have just rolled out of WWII have to fight against 50 ft tall mecha. The bad science often can be just Hand Waved by having it be some Lost Technology or something dropped by Neglectful Precursors. The fact that all technology is ancient Lost Technology just adds to the World Half Empty. There are also references to actual scientific concepts and modern day military designs but not everybody who lives in the 41st millennium do not understand the idea.
Historical RevisionismThe game has evolved through six editions, it has books for each playable army and its own publishing arm for hundreds of novels set in the background universe. It is, in short, a continuity nightmare. So, let's ignore continuity by making half of our background the product of Historical Revisionism. Each army gets to be full of the Rule of Cool, trampling over all other armies because everything you know about them is propaganda. Plus a world without any solid truth gets another layer of GRIMDARK. Now we know who wins in a "vs" fight: both sides! The number of Records Expunged by Order of the Inquisition lets people fill in the blanks as to what really happened. Its adds to the horror of a world where the few good deeds might go forgotten, or where you yourself have to be expunged. It even allows for some comedy- look at Ciaphas Cain, "Hero of the Imperium". Even one straightforward story can be made into several. The legends of the Eldar gods can be interpreted as involving battles between physical entities, Eldritch Abominations in the warp or metaphors for armies using new weaponry.
Actually has something consistent to say about Utilitarianism40K, especially the Imperium of Man, is a fascinating universe in part because it accomplishes something that can only be done when you play some of these tropes perfectly straight. It constructs an Alternate Universe where fascist policies are not just justified, but absolutely required for mere survival. As someone else put it, "if you let me put my thumb on the utilitarian scales, I can get you to agree that you have an affirmative moral duty to torture a three-year-old child to death." Indeed. Consider the humans' position against the 14 characteristics of fascism. Nearly all are justified, because the 40K universe is so damned dangerous. When High Chancellor Sutler says "I want this country to realize that we stand on the edge of oblivion", when Ernst Röhm says that "the people desire wholesome dread", when fundamentalists invoke the phrase "With Us or Against Us", this is the imaginary universe they live in.
40k allows for a deeper understanding of 1984The above entry almost contains this one. Any reader of 1984 comes into it knowing exactly what it is: an exploration of the evils of totalitarianism. There might be some surprises, but you never are fooled into thinking that the Party or Big Brother are the good guys. But what is this? You LOVE the Emperor of Mankind, the eternal savior of humanity, for without his protection mankind would face certain extinction. You know that even heretical thoughts create windows into the Warp for unimaginable horrors, but your moral sensibilities raised by (insert atrocity X) might cause you to question: Is the Imperium's rule really the last, best hope of humanity? Am I fighting for the good guys? Maybe those Tau, or that cult of freethinkers, really work for the Greater Good or want better lives for the miserable oppressed. (the paranoid gut-wrenching moral dilemma here is captured by most half-way decent mindfuck spy movies) Maybe you defect, or are corrupted. You quickly learn that not only are they as bad, but they are worse than the Imperium, that their boot on every human's face, forever, is better compared to alternatives. Hopefully, in the moments before the inquisitorial bullet claims you, you will have the comfort of knowing that YOU LOVE THE GOD-EMPEROR, because HE PROTECTS HUMANITY. If you've been buying the fluff propaganda as truth, hook, line, and sinker, you now have an idea of what it is like to live in the same world as Winston Smith. Minus the whole actually living in a dystopian future.