I have a confession to make. I don't care much for Warhammer 40k. This isn't a criticism so much as it is me being weird and impossible to please, because I love Rogue Trader and it's undeniable that a significant part of that is due to setting elements it inherited from the 40k universe.
The Imperium of Man is a bureaucratic nightmare and a feudalistic nightmare and a capitalist nightmare, and at the intersection of those three tendencies lie the Rogue Traders, a group of peripatetic merchant/soldiers (ie "pirates") who troll the fringes of the empire in search of uncontacted worlds they can ruthlessly exploit for profit. I like it because it's a grandiose sort of villainy where you're a larger-than-life figure, thoroughly enshrined in the trappings of obscene wealth and aristocratic privilege, and your goal is to keep getting away with it.
And all of that is possible because of the Warhammer 40k background. Where it gets a little uncomfortable for me is that WH40K's style of presentation is terribly inconsistent. Basically, it's a mish-mash of influences. Space travel evokes the age of sail, not just in the thoughtlessly mechanical way that most space sci-fi seems to do, but also in its fashions and its military organization and its mercantile politics. Architecture is over-the-top Gothic, even on things that have no business having buttresses and gargoyles (like spaceships). Armor is positively medieval, as interpreted by careless Victorian scholars and with a needlessly inflated scale that reeks of machismo.
The Imperium of Man takes a base of Rome and Byzantium, adds a whole lot of Imperial Brittan, and just a little bit of Nazi Germany and the worst parts of America, and then seasons with a very superficial parody of Catholicism that is so extreme even Bill Donohue couldn't object. It's like the Golden Age of White People, filtered through the most fevered imaginings of the "European Identity" assholes. A lot of the time, it is self-aware and satirical and you get a proper sense that the Imperium is transcendentally awful - it is as corrupt, as unjust, as inefficient, as ignorant, and as self-defeating as a society is possible to be, its power and its grandeur existing only to expand the scope of its failings into the ludicrous.
When it's most like this, Warhammer 40k is actually pretty good, but sometimes it seems to get a little . . . lost. The text forgets that the Imperium is supposed to be a joke and revels in its aesthetics and its military might. You're playing space fascists, but you're encouraged to be awesome instead of ridiculous, and when you catch that happening, it's always a little . . . ew.
The practical upshot of this is that a book like Rogue Trader becomes a paradoxical minefield. It's ostensibly informative, but it's got an unreliable narrator. It leaves open questions about things that would be useful for running a game (like what sort of technolgy exactly, the cult of the Omnissiah finds heretical) and you can't be sure if it's an oversight, an artifact of trying to sum up 30 years of ever-evolving and highly complex canon in a book of finite length, deliberate vagueness, or a joke about how dogma-driven technology has become.
A good example of this is in the sample adventure. You're looking for a derelict treasure ship containing the plundered wealth of an entire world, and the world in question is described as "a world of vast wealth and blasphemous grandeur."
I can't help wondering whether Krystallian was blasphemous or merely "blasphemous." In WH40K, you can deviate from imperial dogma by embracing ideas like free thought, scientific inquiry, social equality, and accountable government OR you can deviate from imperial dogma by worshiping a minotaur who wants to drown the universe in blood. The Ecclesiarchy reflexively conflates the two because they are the dumbest motherfuckers ever to live.
Warhammer 40k is at its best when it nods its head and agrees that these are, indeed, some dumb-as-hell motherfuckers, but the colossal monument they've built to their own wrongness is so vast and intricate it will take millennia to collapse under its own weight. It's at its worst when it tries, straight-faced, to say, "no, the metaphysics of the setting are such that democracy does lead inevitably to demon-gods from the realm of nightmares devouring your whole planet."
I love Rogue Trader because it's aggressively and gleefully a sort of nega-Star Trek. Your continuing mission is to seek out new life and new civilizations and steal all the shit that isn't nailed down. And then steal the nails. And then steal all the shit that used to be nailed down. You're basically what the Ferengi should have been in TNG - a force that blurs the lines between commerce and militarism and puts the lie to the unctuous niceties of imperialist capitalism.
When I think about the game in those terms, it makes me wish that Rogue Trader could have included among its adversaries a society of galactic do-gooders, some kind of Federation expy to act as a foil for the PCs' campaign of ruthless exploitation. Maybe not large, but technologically advanced and far enough away from the main body of the Imperium that it would not be instantly annihilated. Unfortunately, this is the 40k universe, and thus the only people who ever survive long enough to become a serious threat are the absolute bastards.
All this really means is that when I play Rogue Trader, I tend to do it off-book. Yes to playing the villains, but no, their enemies aren't always conveniently worse.
As far as the game itself is concerned, I mostly like it. It's not going to be remembered for any great innovations, but in a way, that's comforting. At its core, it's a simple percentage-based system that uses a lot of very conventional mid 2000-s ideas about things like player agency and action economy. It could probably be converted to d20 with very little loss of fidelity and in fact does that thing that D&D 5e does where it keeps the bulk of 3e's tactical complexity but pretends the grid is a needless imposition and tries to do everything "theater of the mind" style without actually abstracting away the fiddliest parts of the system.
When I read that the Overwatch action allowed you to take a shot at anyone entering a 45 degree arc in front of your weapon, I spent about an hour trying to figure out a simple geometric shortcut to let you define the cone from the midpoint of a square's edge, but I wound up giving up when I realized I'd need a compass to bisect angles. And that might seem like a mark against using a grid, but the alternative of doing it purely in the imagination means that you're eventually going to wind up having to be just a little bit whimsical in deciding which enemies get successfully Overwatched by your players.
Rogue Trader does do a couple of interesting things, though. Character advancement is done through a hybrid point-buy/class system that might have potential (basically, your class and level determines what you can buy with your points), but which in my limited experience almost instantly resulted in players trying to subvert it through optional "elite advances."
The other thing I liked, despite the fact that I've never encountered the necessary level of player buy-in, was the Endeavor system. The short and sweet version is this - Players in Rogue Trader start off super rich. So rich, in fact, that even the "poor" ones are better served by an abstract resources system that replaces detailed currency accounting with a simple percentage roll. But the goal of the game is to get even richer than that. Endeavors are an attempt to put your wealth-gaining activities on an objective footing by tying them to an adventure-pacing mechanic that can be influenced by characters' actions. The players say that they want to set up a new trade route and then the GM gives them a list of Objectives - short adventures - that if they accomplish them in suitable style will indicate that all of the tedious background stuff also got accomplished in the meantime.
It's a good idea that gives players a role in defining the narrative, but it's also a style of play that will take a bit of getting used to in an otherwise very traditional rpg.
To sum up - Rogue Trader is far from perfect, and there are many difficult and dangerous ideas that need to be negotiated to get the optimal use out it, but sometimes it can be fun to play as a crew of self-serving miscreants, and few games do that with quite so much style.
Ukss Contribution - I'm going to be a coward here and shy away from the more distinctively 40k setting elements. Instead I'm going to pick a small thing that I thought was pretty neat - the fractal blade. It's a spiky sword, but if you zoom in on the spikes, it turns out that they are themselves miniature spiky swords. And those spikes are also super-small spiky swords, and so on, forever. If you break off one of the spikes, you can plant it in the soil of a certain world and in time it will grow into a whole new fractal blade.
It's not world-shaking, I know, but I like the image.