Thommy H:
I didn't know anything about the 'Star Wars' universe until I saw the movie
But the Star Wars franchise is 99% the movies... Watching A New Hope is how you're
supposed to be introduced to the universe. 40K is a setting that already exists, and it's one which is not immediately accessible, even when playing the game for the first time. It's potentially quite immersive, complicated and non-intuitive (there are no clear cut heroes, it's is a cross between sci-fi and fantasy, there are a lot of mythological references, it's intentionally over the top, etc. etc.) and if the object of this movie is to
introduce people to it (and it must be) then you don't do that by pitching straight for the darkest, grittiest corner of it. No one's going to "get" 40K if the first thing they see is some incredibly dark, gothic horror adventure that doesn't even involve any spaceships or guns or aliens. That's not "40K". It might be the most interesting part of the universe, but so what? They need to be promoting the most recognisable hook they can with this project, and that means Space Marines, and it probably means Ultramarines too.
To use one of my earlier examples: if you want to get into Doctor Who, you don't start by watching Torchwood. Both might be decent shows and, for all I know, Torchwood may be the stronger franchise (I know next to nothing about Doctor Who...) but it won't tell what you need to know to understand the setting. You need to get the basics before you can start to understand the difficult bits. So, with 40K, it's no good starting with something as nebulous and confusing as the Inquisition. For one thing, the exposition would be way too difficult.
Narrator: Right, there are these shadowy agents of the Imperium...
Viewer: What the Imperium?
Narrator: Well, it's this galaxy spanning empire that...
Viewer: Oh, so this is sci-fi then?
Narrator: Yes, and it was founded by the Immortal Emperor...
Viewer: How was he immortal?
And so on. You start with Space Marines, and everyone gets it right away, because they know what a Space Marine is (they've seen Alien and played Halo), and then you can play with that trope and, pretty soon, people get the blend of the gothic and the eldritch that makes 40K what it is. "Master Chief, if he was like a medieval knight" pretty much explains the whole setting.
Anyway, despite arguing all this, I haven't even watched the trailer and I think this "movie" will be a total piece of crap.