Ah, thread no. 2 for today. I don't know if this is totally appropriate in this forum but buggered if I know where to put it
I've just been thinking, the difference between WHF and 40k as works of fiction is not that great. 40k Rogue Trader was created as fantasy in space as you all probably know, but since then, 40k has arguably become more popular than WHFB. Still, they are almost the same thing, and I think not in the way the writers back in the 80s intended. I'd classify them both as forms of dark science-fantasy with steampunk elements.
They both have demons and magic, which are major tropes of fantasy fiction. They both have guns, which signal science fantasy. The guns are clunky, unreliable, and poorly understood by our standards, which is a common trope of steampunk. The guns even look old-fashioned (guns in WHF are black powder, Guard tanks in 40k look like they are from a hundred years ago).
They both have non-human intelligent bipedal species (fantasy again, or at best science-fantasy), They both have mythologies essentially based on polytheistic creator gods and a war in heaven, more fantasy tropes. They both place humanity on the back foot, facing unspeakable threats using weapons and magic that they barely understand (dark fantasy). I'm sure the list could go on.
My point (yes, I do in fact have one) is that really, the major GW universes do not just share similarities, they are virtually the same thing. Even the dark tone is the same. The only real difference between them is the outer clothing of the tropes: 40k wears a dark future dress, and WHF is dressed up all medieval.
Sorry if everyone already knows this, I was just kind of shocked to realize that we normally think 40k is scifi, fantasy is fantasy, but I think it's more accurate to say they are both science-fantasy, just different.