Falling into an unusually expansive set of genres, Warhammer 40,000 is best described as "fantasy in space", except when it's "horror in space" or "World War 2 in space" or "World War 1 in space" or "1800s naval battles in space" or any of those things on a planet instead. To put it bluntly, Warhammer 40,000 (or "40k") is all over the board when it comes to believability. The universe is so huge that it's grounded in some areas, unlikely in others, and straight up ridiculous in a lot. Because of my specific focus on grounded elements, this analysis will be focused on the Imperium, with other races (Orks, Eldar, Tyranids, and so on) treated as threats or adversaries.
Warhammer 40k takes place in the distant future, after humanity has expanded to the stars, experienced a cataclysmic apocalypse that left all its colonies stranded and isolated, and gradually built itself back up with the help of a near-divine psychic leader who ended up sacrificing himself to the point where he exists only as a navigational beacon for psychic ship pilots to traverse the gulf between the stars. So, let's back up a bit. 40k's dense backstory can be summed up in a few key ways:
1: 40k's universe is like our own, with the exception of the Warp, which spawns demons, drives people insane, and also is responsible for some individuals becoming psychic. The Warp is used as hyperspace, which is unpleasant and unreliable and results in there still being a great distance between planets (compared to, say, Star Wars).
2: There's a lot of planets that have humans on them, but have been removed from the rest of humanity for so long that they're basically primitives and/or have built themselves back up to medieval/industrial levels.
3: Most of the technology used by the Imperium is built from modules developed during humanity's spacefaring days, which creates a sort of constant pattern across the Imperium despite the distance between planets.
So, in short: humans are separated by the vast vacuum of space, and are only able to traverse that gap at great cost. The isolated humans have thus created many different cultures, but are unified by their standardized technology (which they don't really understand enough to improve upon). In gameplay, this is reflected by the fact that human forces (the Imperial Guard) use basically the same gear, but have a lot of differences in terms of their abilities and specialties. There's standard troopers, jungle fighters, arctic fighters, desert fighters, and oddities like the Vostroyans, the Mordians, and the Attilans. Each has rules in gameplay for their specialty, and - more importantly - these rules are based off a point-buy system, making it possible for the player to make up their own planet and regiments.
Despite its logical inconsistencies in a lot of fields, 40k does an excellent job with the Imperial Guard. Heck, there's an entire book dedicated to describing the gear, weapons, and procedures of the Imperial Guard - and that's all their gear, including mundane things like communication gear, rations, gas masks, and weapon cleaning kits. In that sense, Warhammer 40k is probably the most detailed example of "World War 2 in space", at least in terms of pure detail. Gundam and Star Wars, the other two major contenders, both had to deal with inherent limitations in tactics, but 40k has pretty much made a total transition: it's World War 2 tactics and gear, but sci-fi.
Ecclesiarchy, a pervasive state religion based on the worship of the Emperor (the super-powerful psychic responsible for bringing humanity back to its spacefaring state). Like the standardized gear, the Ecclesiarchy establishes a consistent theme throughout the Imperium: that of Emperor-worship. While the ways in which this is accomplished differ (ranging from institutionalized and organized religion to missionaries and pagan conversion), the underlying faith of the Imperium is pretty much standard. This is also a source of greater motivation - the wars fought by the Imperium are not just for defense, but are righteous crusades as well.
Despite being a minimal part of the series in terms of their usage (as in, 99% of the battles are fought by the Imperial Guard), the Space Marines are one of the most common factions due to their popularity. As mentioned last update, the Space Marines bring a lot of logistical concerns to 40k. While the Imperial Guard are recruited en masse as cannon fodder, the Space Marines are trained and mutated from the best of the best and indoctrinated to their very cores with faith in the Emperor. There are roughly one million Space Marines in the galaxy, compared to trillions of guardsmen. However, on the tabletop, they're not that much better than a guardsman, for balance reasons.
This results in a huge power variation: on the one hand, they're unstoppable killers mounted in the equivalent of tank armor and equipped with a fully automatic RPG. On the other hand, they're better-than-average infantry in nice armor with a machine gun. The background material matches the former, the gameplay matches the latter, and combining the two usually ends badly. There are some things that the Marines are better suited for - hunting monsters, cleaning out abandoned ships and facilities, and engaging powerful or unorthodox foes - but the game naturally shoehorns them into a bunch of standard infantry roles that they're undermanned for (and for which their armor and bright colors are nonsensical).
There is some diversity among the Space Marine chapters, though not as much as the Imperial Guard. There's Roman Marines, Viking Marines, Mongol Marines, and so on. Like the guard, the gear is generally standardized, but unlike the guard this includes their armor (which constitutes 90% of their appearance). However, their armor is actually kind of key to their popularity: its large parts, including the highly notable pauldrons, make it fairly simple to paint - while still allowing for customization in the form of color schemes and insignias.
While the non-Imperial factions have a lot of attention given to them in terms of backstory and individuals, they lack the same diversity that the Imperial Guard do. In some senses you can look at the entire story as being largely Imperial-centric: the Imperium gets the most attention and development, and other races exist to react to, or attack, the Imperium. These races include the Eldar (space elves), the Orks (space orcs), the Tyranids (bugs), the Necrons (space undead), and Chaos (demons and demon-worshippers). None of these races get the same diversity as the Imperial guard; there's different clans and sub-groups among them, but this usually translates to "different colors and one thematic thing".
Why is this? It could be argued that the conditions of the Imperium make it better for gameplay-balanced diversity. Other races can have different things, but without the framework of "religion + technology limitations" there's no reason why they should be so homogeneous. It could also be the standard "aliens are all alike" concept - there's differences among the Eldar craftworlds, for example, but this is limited to minor themes and specialties. Their equipment (including armor, the primary visual signifier) is entirely the same.
In addition, the general "theme" of humanity in 40k is one of constrained adaptability, making them a balanced choice. In contrast, each alien species has their own theme - Eldar are sneaky, Orks are brutish, Tyranids are innumerable, and so on. While there's diversity among their troop types, their general theme cannot be deviated from without changing the species as a whole. This reflects a sort of human-centric concept: humanity are the protagonists, aliens are the diverse invaders that serve as their opponents.
Unlike a lot of game franchises, "Warhammer 40,000" is actually a pretty gameplay-diverse game. While the original squad-level wargame is the most famous, there's also a space combat game, a gang warfare game, a spaceship clearing game, and RPGs of the high-flying (Rogue Trader) and grim and gritty (Dark Heresy) persuasions. What does this do for a universe? There's rules for pretty much every part of the universe except, perhaps, civilian life - and even that's fleshed out by the RPG for reasons of necessity. When I analyzed Lost Planet, I said that it would work better as an RPG so that gamers could take advantage of the established universe in ways that an action game would not. I feel similarly about the Killzone franchise: there's a lot of neat things that I think are wasted in a simplistic FPS.
While there are a lot of things about 40k that don't really add up, the diversity of games means that there's at least some attempt to flesh out the background out of necessity at the very least. A similar thing happened with the Star Wars RPG by West End Games - the universe had to be explored in-depth so that players could interact with it. The smaller necessary details add up to a larger experience, although keeping them consistent is another matter. In the 40k universe players can be everything from the lowliest thug to the highest general, and that does a lot for a franchise in terms of establishing the scale of events. As I've said before, every large concept is made up of smaller ones, and by analyzing these more personal aspects of the setting, Warhammer 40k has fleshed out its background rather nicely.
One of the most controversial aspects of Warhammer 40,000 is its nature as being "grimdark". In essence, the 40k universe sucks, period. There's religious oppression and governmental oppression to the highest possible degrees, anyone can die at pretty much any moment, and thinking too hard about free speech will cause demons to possess you (if the inquisitors don't get to you first). As always, this is handled in different ways by different authors, but let's start with a base assumption: things are bad in the 40k universe because there are reasons for it to be.
A lot of the more (and less!) "grimdark" examples come entirely from author fiat. The baseline standard for things being awful comes from the universe itself - there's demons, monsters, and aliens, and all of them are hostile for their own reasons. However, there are some cases where things are either worse than normal or better than normal, not for any explained reason, but because it's thematic. The former can lead to Darkness Induced Audience Apathy, because every faction is bad. The latter, as represented primarily by Ciaphas Cain, can result in more audience attachment - but can also undermine the established reasons for everything else being unpleasant.
To me, the latter is worse than the former. When things are worse than normal, there's always a person for the audience to be sympathetic with - and that is the people who are suffering. No, there's no "good" faction, but that doesn't mean there aren't people who are being exploited both by their allies and their enemies. When things are better than normal, however, there should be a good reason for it (and sometimes there is). I singled out Ciaphas Cain because there are a lot of things that he gets away with that he really shouldn't, and the reason that this is allowed is because of author influence. The Ciaphas Cain books are popular because they're less depressing than the standard 40k fare, but at the same time I feel it's important to establish that they're not operating under the same rules. Ciaphas Cain is a snarky adventure novel that happens to be set in the Warhammer universe.
A lot of fantasy is "lighthearted", but it only gets that way because it ignores a lot of the realities of medieval life. When someone dies, it's appropriately dramatic and sorrowful. Protagonists are safe from harm. It's escapism at its most protected - and 40k is almost the opposite of escapism, because the parts of real life that it connects to are the depressing parts that we try to avoid. The same is true of Warhammer Fantasy. It's not unrealistically awful, it's just that in most cases people don't actually know how bad it was during those time periods (and how much worse it would be with the addition of orcs, demons, and so on).
This can lead to two characterizations of the fanbase in general: the "meta" perspective ("this shouldn't be so depressing, why can't I have a fun, lighthearted wargame") and the "immersive" perspective ("look at how many people just died, how can this not be depressing?"). Both sides are complicit in the violence, at least in-universe, but one group doesn't want to feel bad about it, and the other group recognizes that war is generally unpleasant for the people involved in it. What bothers me the most about Ciaphas Cain is that people still die - it's just expendable extras instead of protagonists, and that means it can still be lighthearted so that Cain can make some wacky quip. This is not a perspective that I support.
The things I don't like about 40k are unconnected thematic leaps. I don't like that the Space Marines (and most of the alien races) are so inconsistent in terms of their power. I don't like attempts to either darken or lighten the setting that have no logical basis to them. I don't like, but can accept, the fact that most of the planets are tiny and single-biome. I don't like the clash between "thematic tactics" (stand in the open wearing bright armor) and "sensible tactics" (take cover, use fire support and camouflage).
To me, 40k is at its best when everything connects together: humanity is in its current state because of all the factors that led to it. 40k's design decisions create a lot of dynamics that make it unique, despite using a lot of standard sci-fi concepts. Humanity in 40k is a fascist theocracy that's forced to scavenge technology and is susceptible to the influence of demonic forces. In addition, humanity itself is spread out among countless planets and has been forced to re-adapt new cultures into the larger collective society. This is a setup that basically no other setting can match - they might play with similar concepts, but none of them have that same "big picture". This is why I'm willing to tolerate 40k's bad parts: because its basic treatment of humanity is so interesting and so tangible.