To a certain extent, my taste in fantasy might be shaped by the fact that I never had a very religious upbringing. While both sides of my family had Abrahamic traditions (one Jewish, one Catholic,) there was never a strong pressure to really conform to either of those. While not religious exactly, I've always been fascinated by the philosophical distinction between the mundane and the supernatural. In a way, that's kind of distinction between science fiction and fantasy. The best I can do to define the two is that in fantasy, the supernatural is ultimately mysterious. If you can sufficiently analyze magic, it just becomes a new set of physical laws. But in a science fiction setting, you can propose beings of such tremendous power and sophistication that, to us humans, their technology can only be appreciated as a sort of magic.
It's an interesting philosophical question whether technology and magic are inevitably on a spectrum and that the only distinction is our understanding of it or if the two are fundamentally different things.
The Warcraft Universe has plenty of magical stuff in it, but I actually think that if we consider this distinction (whether magic can ever be truly understood) the difference between Sci-Fi and Fantasy, then Warcraft actually leans a bit into the science-fiction side of things.
I'd hesitate to say that it's truly science fiction, though, because there are some things that remains mysterious. Much like Star Wars, it fits into the hybrid "Science Fantasy" genre, though Star Wars, having a more traditional science fiction setting, is generally thought to be from that genre, whereas Warcraft's setting is more similar (at least at first glance) to the Tolkien-style grand fantasy tradition. But unlike Tolkien, where there is a very clear creator of the universe who is ineffable, no individual in the Warcraft universe appears to be entirely, independently supernatural.
That's arguable, of course. But if we try to come up with the most powerful figures in Warcraft, the best answers are probably the Titans and the Old Gods.
The Old Gods seem to come from the Void, but they aren't void beings the same way that Voidwalkers are (the Voidwalkers could potentially seem "inherently" magical, but unlike the Old Gods, I suspect they are not really sentient, living beings.) The Old Gods seem to actually be organic in some sense, which suggests they are less truly "gods" than really Lovecraftian aliens. They are so fundamentally different from the animal life that we Azerothians are familiar with, and despite not being humanoid in appearance, are clearly far more advanced and intelligent than any mortals on Azeroth. They use void magic in a way that we could never hope to, and this makes it seem as if they are truly one with the Void - but I don't actually think they are. Instead, they are channeling the power of the void in such a sophisticated way (perhaps a way that has become part of their physiology to the extent that they don't even need to consciously "cast" shadow magic) that a mere mortal can only look upon them with awe (and madness.)
On the other side, you have the Titans, who almost go out of their way to demonstrate that, while incredibly powerful, their power is based in a scientific, technological understanding of the universe. They use magic the same way we use electricity. It's an understood force that behaves in ways that are predictable if used correctly, and they engineer with it. It seems as if they even might use the Holy Light as one of their tools.
Demons, it appears, are simply races that were once mortals and were inundated with Arcane energy (something that mortals don't really understand, but the Titans probably do.) The original demons were mutated and addicted to the Arcane, and Sargeras somehow altered this energy to create Fel, which empowers his Burning Legion to be even more dangerous and destructive than the demons he was originally fighting.
Finally, Naaru seem to be intelligent while also fundamentally attached to the Holy Light (tied to it just as the Voidwalkers are to the Void,) and that's another place you could say my argument breaks down. That's assuming, of course, that we won't discover in the future that someone (perhaps the Titans) created the Naaru. Really, explaining the Naaru as essentially magic robot angels would make more sense than any other explanation I could give you.
One of the key ways in which Warcraft seems to exist in a broader, secular worldview is that there might be demon lords or "gods," but they aren't inherent to their domains. Nozdormu was, for all intents and purposes, the Azerothian God of Time for most of his life, but it isn't like time itself wouldn't run without him. Instead, it was a kind of ministry position. It's like the distinction between being a Fertility Goddess or being the Secretary of Agriculture.
Also, while there are important historical events that are alluded to, there's no real official "beginning" of time in Warcraft. Even if many, most, or even all of the playable mortal races were ultimately created by the Titans (and then mutated through various things like the Curse of Flesh or later the Plague of Undeath or the Worgen Curse,) there's no "In the beginning." We still know very little about the Titans and how they came to be. We don't know how the Old Gods came to be. We're only just starting to scratch the surface on how the Demons came to be. But the universe itself plausibly could have come about the same way that modern physics believes ours did - a Big Bang, particles coalescing into atoms and then stars and planets forming, and then some unlikely chemical combination that could replicate itself and mutate and eventually become what we recognize as life.
Diablo is very different.
First of all, it's binary. There are really only two sides to the supernatural coin - good and evil, angels and demons. Outside of Sanctuary, as far as we can tell, there's really nothing other than those two types of things (though I do think there's some sort of mystery about dragons.) The Angels and the Demons have been waging war against each other ever since they arose, and they were both created out of the remnants of the ur-Good, Anu, and the ur-Evil, Tathamet.
The Nephalem, and their human descendants, introduced moral ambiguity to the universe. Born of both angelic and demonic ancestors, the Nephalem had the potential for both good and evil, and were actually practically Gods.
And that's, I think, the saving grace of Diablo's lore. With good and evil being so strictly separated, it becomes a pretty uninteresting cycle of the same things over and over. But in Diablo III's expansion (that made the game good, finally,) we got to see some very interesting twists that made good seem evil, or perhaps evil seem good?
Humans are half-Angel, half-Demon. Demons are, fundamentally, evil. At the end of Vanilla Diablo III, all seven of the Lords of Hell are trapped within Zoltan Kulle's black soulstone. Thus, it would seem that the only remaining evil in the universe is the demonic half of each human. Malthiel, the Angel of Wisdom, goes down to Sanctuary to retrieve the Soulstone and attempts to use it to siphon off all the demonic souls in Sanctuary - which would literally involve tearing every human's soul in half, annihilating them.
What's really fascinating to me about this is that in one sense, if you think of evil as a real thing that has a kind of tangible substance to it, Malthiel is right to do what he is doing. He has an opportunity to rid the universe of all evil.
But in a much saner way of looking at things, it's clear that what Malthiel is doing is evil. Evil is not some substance that can be cleaned away. It is cruelty and callousness and violence against the innocent.
And ultimately, this casts the entire Eternal Conflict into a kind of new light. Before there were humans - specifically, before there were mortals - what really made one side of the Conflict better than the other? Each sought to destroy the other, and each had its own way of doing things, but outside of the context of day to day human morality, how could we even seriously call one side good and one evil?
Once presented with humans, Angels and Demons were given an opportunity to demonstrate the value of their side. Angels for the most part seem to be a lot nicer - Tyrael in particular, but most of them seem to be capable of sympathy for us.
But Malthiel did not show that, and his actions made him something just as evil as anything that came out of the Burning Hells.
I don't know what the future plans for the Diablo series are - whether they're going to do another expansion or start working on Diablo IV - but Malthiel opened the door to much more interesting stories. Then there are the Nephalem themselves - reborn into the world, we might see the old good/evil dichotomy break down as new Gods - more powerful than the Archangels or Demon Lords of old - start to throw their weight around.