Somewhat like Dungeons & Dragons' two "just outside our reality" realms, the Feywild and the Shadowfel, World of Warcraft Chronicle has revealed two realms that are likewise just barely outside the physical plane - closer to our realities than even the Elemental Planes.
One that should be pretty familiar is the Emerald Dream, which is affiliated with the Life Force and is basically a reflection of reality that is full of lush nature, unblemished by the work of humanoids. The Emerald Dream is under assault by the Nightmare - at this point I don't know if we have a definitive source for the Nightmare, but there could well be an answer in Chronicle. Still, while it is a darkened version of the Dream, as far as I know, it's not truly the opposite.
That opposite is the Shadowlands. And believe it or not, you've been there. Probably many, many times. In fact, I'd guess that it's the alternate realm of existence that players most frequently visit outside of the mundane, physical plane.
The Shadowlands are the lands of the dead.
Every time that you die in-game, you find yourself in an empty, desaturated version of wherever you had been. If you look up, there's a swirling vortex in place of the usual skybox. There are Spirit Healers in this version of reality (in terms of mechanical design, this is the most common form of phasing,) but you'd be forgiven if you'd always assumed that this was just a gameplay mechanic without any lore behind it.
And that might have been the case. Originally.
But now it's pretty clear that that ghostly version of the world that you make your corpse runs through is actually The Shadowlands. Those Spirit Healers? They're actually Val'kyr who do their best to find the souls of the dead and help them return to the land of the living.
So that's right, every time you've made a corpse run can now officially be part of your character's canon. It's lore-consistent.
Not only do we now have one of the most common things in the game given a rather elegant lore justification, but we also have some new opportunities for lore-based mechanics. Death Knights, for example, are getting a new ability called Wraith Walk. Likely based on Leoric's ability in Heroes of the Storm, it allows the Death Knight to step into The Shadowlands, giving them a speed boost and breaking snares, but being unable to take actions until it's finished.
I'm really eager to see the graphics associated with this ability. With Demon Hunters' Spectral Sight altering the entire look of your screen when you use it, it would make perfect sense for Death Knights to view the world as it looks while you're a ghost when using this ability.
Anyway, while I'm slightly saddened to have a lot of the mysteries that are fun to speculate about getting definitive answers (some of those answers painting a rather bleak picture of the Warcraft Universe,) there's also so much more fuel for speculation. The first book covers things only up to the birth of Medivh, so we'll have to wait to get more definitive answers about just what the Lich King really is, but I'm definitely excited to learn more.