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 Evil Versus Evil. 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 Angels 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 in 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 Crapsack World. 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 eight 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.
World of Badass for all humanityA large part of the draw of Warhammer 40K is that, perhaps more than any other setting out there, it portrays humanity as a supremely badass race. In Warhammer 40K, humanity is thrown into a world packed full of insanely powerful threats, all of which want to annihilate us. We must deal with omnivourous living weapons more numerous than the stars, invincible omnicidal Terminators, Ax-Crazy Reality Warper fungus aliens who grow from the very ground and attack us for literally no reason other than that they're programmed to do so, Space Elves who are physically required to inflict torment on humans in order to survive, and demon-possessed Super Soldiers. Only a theocratic paranoia that makes the Spanish Inquisition look like a paragon of tolerance keeps The Legions of Hell from breaking through into the real world, and even still there are frequent leaks. To travel between stars one must take a shortcut through Hell itself, guided by a psychic beacon kept alight by the daily sacrifice of a thousand souls.Humanity is dropped into this setting, left to fend for itself, and we survive. Despite all the nightmarish perils seeking to snuff us out, we survive. If humanity can survive this ultimate Crapsack World, what can we not achieve?
The Tau as an examination of Humans Through Alien EyesAs a race, besides the Tyranids, the Tau can be seen as Giant Space Flea from Nowhere in the 40K setting, considering that unlike almost every other race, they have no connection whatsoever with the ancient War in Heaven between the Old Ones and the C'tan, and are total new comers to the galactic stage.
When you think about it, the Tau are basically the embodiment of every single trope/clichés that humanity usually have in most space opera and sci-fi stories.
Just look at the list: The youngest and newest member to the galactic stage, idealistic and wish to explore the galaxy, prefer to use diplomacy when dealing when alien races, have a quick rate of technological and social advancement when compared to everyone else in a stagnant galaxy, being the leaders of a multiracial coalition... etc. Except that this time, these tropes are not embodied by the humans in the setting, allowing us to examine them without our innate bias towards our own species getting in the way.
So the next time you see an Imperium fan dismissing or hating the Tau for being annoying and naive, you are getting a perspective of how humans look through alien eyes in most other sci-fi settings. You will finally understand why the Romulans and Cardassians fight The Federation in Star Trek, or why the Asari look down upon humans as brash and ignorant in Mass Effect.
Wishy-washy Canon: Death of the Author as a component of Warhammer 40,000's universeLet's say, for a moment, that you and your friends are arguing about who the good guys are (or at least the "good" guys) in 40k. You think the Tau are the good guys, one of your friends thinks the Imperium of Man and its subfactions are the good guys and the other thinks the Eldar are the good guys. Throughout the argument you extol the virtues of your favourite faction while listing the flaws that disqualify the other two from calling themselves good guys.
So who's right in the end? By the standards of this game's canon, you all are.
It's very easy to claim that any one side is being dracoed by a Misaimed Fandom, but most of them, if not all, have at least one virtuous or redeeming trait. For the Imperium, it's that a lot of the groups they fight really are threats to them; for the Eldar, it's that they fight Chaos and the Necrons even more fiercely than the Imperium, and are willing to join forces with others to fight both; the Tau genuinely want to make the galaxy a better place, the Orks display a lot of Villainous Valour, Nurgle and his followers provide a caring and human element to Chaos and so on. This doesn't necessarily mean that any one side can objectively be called "good guys", but saying that the entire galaxy is nothing but a Crapsack World dominated by Evil Versus Evil is a very pessimistic view.
The thing is, with Games Workshop having stated as Word of God that all codices are in-universe propaganda so they can scrub anything the fans really don't like, they've surrendered a lot of authorial agency. Any misdeeds they write into a faction's background to correct players' view of them can be Hand Waved as malign historical revisionism, anything good written about another can be read as being biased. This is further fuelled by player-made armies, which are treated as almost canon, but not quite and which are only as noble or corrupt as the player decides they are. If someone wants to create armies and/or alliances which would never be formed in-universe — a Tau hunter cadre with an order of noblebright Sisters of Battle serving as elite Gue'vesa auxillaries, a joint Eldar-Necron colonial defense force, a World Eater warband who are non-evil Proud Warrior Race Guys with Thousand Sons allies and an Ork clan who serve as the Chaos Lord's elite personal guard or even a cabal of Dark Eldar who breed Tyranids, Monstrous Creatures especially, as beasts of war — they totally can. Imagination and personal interpretation of the fluff are the only things holding them back.
This serves to create a situation akin to Schrodinger's Cat (Schrödinger's Canon?), where the canon is technically in several different states at once because everyone's interpretations, short of the seriously out-there ones, are all valid to a greater or lesser extent. For this, Games Workshop have only themselves to blame.