I only have a passing knowledge of Warhammer 40k, but isn't it Starship Troopers taken to the max? Space fascists, human sacrifices (to keep the Emperor alive) etc etc? And then there's the horror aspect: servitors, flying skulls, whatever the flying infant-like things are called, etc etc; never mid the Tyrannids. And I'm struggling to see how they could keep all that sufficiently attractive.
I think it's been described as a mash up of pretty much all the sci-fi genres.
You've got nods to Terminator, Starship Troopers, 1984, Aliens, cyberpunk, etc etc.
Mix in copious amounts of horror, some humour to taste.
There are major 40k stories that are very much "ww1 battlefields, but with lasers and the odd demon" or "aliens, but the human faction have equipment ranging from little more than a butter knife and a magazine stuffed under the shirt for armour, to the humans are wearing tanks", to "giant robots battle, RAH", to "terminators, but they won't stay dead" or "space elves who might go from being your enemy one minute, to fighting by your side the next", to "battle of Rutland, in space, in ships the size of cities"
As a couple of others have said, they don't have to introduce it all at once any more than you had to know about 50 years of comics to go in and watch Superman or Guardians of the Galaxy, and there are at this point probably tens of millions if not many more who are reasonably aware of it either because of playing the table top game (or having friends/family who have done), having played the video games or read some of the books.
