As previously mentioned, the new World of Warcraft Chronicle Vol. 1 is giving us a really broad and comprehensive sense of the lore for the series. We know why the Titans are interested in Azeroth and a timeline for the reigns of the Elemental Lords, Old Gods, and Titans on the planet, all from the 20 preview pages made available. But I think a great starting place for talking about the lore is this very beautiful cosmic diagram:
As someone who is a (rather new - only started a couple months ago) DM in a Dungeons and Dragons game, this is actually pretty familiar - multiple layers of domains and realities that expand outward from the familiar, material world.
Starting from the outside, we have six major powers that seem to be opposed to their opposite numbers - yet those who are next to each other are certainly not allies either. You have the opposing forces of Light and Shadow, Life and Death, and Order and Disorder. If you draw a line between the upper right corner and the lower left, you kind of create a line dividing good and evil - at least until you get down to the next level, which contains the elements and an interesting other pairing.
Each of these major powers of the Warcraft universe has a representative type of magic. The Light, unsurprisingly, has Holy magic. Disorder has Fel. Death has Necromantic magic. Shadow has... shadow (though it's also referred to as Void magic.) Order has, interestingly, Arcane magic - which previously had been talked about as a chaotic, corrupting force. Life has Nature magic.
Subsequently, the various powers and types of magic have a kind of prime representative. The Light has the Naaru. Disorder has the Burning Legion, and presumably Demons as a whole (and at this point, we can probably assume that any demons in the Warcraft universe that haven't been enslaved by Warlocks are probably affiliated with the Legion or with the Illidari.) Death has the undead - broader, certainly, than the Scourge, but there's obviously no better representative of the undead in our experience of the Warcraft universe. The Shadow actually has two representatives. Closer to the pure, magic side of things, there are the Void Lords - which seem to arise when a Naaru dies, though I wonder if the reverse process may in fact be possible. But the Shadow also has the Old Gods, who seem to be tied more closely to physical reality. Order has the Titans (and I don't know how much to read into the fact that the Titans are next-door neighbors with the Old Gods on this chart - that said, its possible they spent more of their history dealing with demons than Old Gods - making the Old Gods the Joker to the demons' Ra's Al-Ghul.) Finally, Life is represented by the Wild Gods - a blanket term for the Ancients worshipped by the Night Elves, the Loa worshipped by the Trolls, and the Celestials worshipped by the Pandaren (and we can also probably throw figures like Lo'gosh, the wolf spirit worshipped by the Orcs who probably just is Goldrinn as well.)
Here's where things get a little trickier. I don't know how much to associate the various elements with their nearby Major Powers. I totally get that Spirit should be near Life (it's the Shamans' "Fifth Element" that Monks refer to as Chi) and Decay (which is really kind of a lack of substance) totally makes sense to be near Death. Earth being affiliated with Order and Fire with Disorder also makes a decent amount of sense. But Air being affiliated with Disorder and Death? Water with Life (ok, yes) and Order?
Spirit and Fire both seem to be in the Light's pie-slice, which actually makes decent sense. The Shadow then gets Decay (yes) and Earth (...maybe?)
It's possible that the primordial elements aren't really meant to match up precisely with the Major Powers, but perhaps they are meant to. I just don't really know what to make of the oppositions between these.
The next layer down is simpler and kind of easier to understand, but it also opens up a really interesting new realm of possibility for settings. Just one step above reality, we have the Emerald Dream and the Shadowlands.
Shadowlands, you ask? What the hell are the Shadowlands? One moment.
Obviously, the Emerald Dream is oriented toward Life and the Wild Gods, situated between Spirit and Water (water brings life.) The Emerald Dream is a kind of platonic ideal of the natural world, and we know that unlike the Elemental Planes, it is a direct reflection of Azeroth - or at least a version of Azeroth that exists as if there had been no civilization or intelligent peoples.
In Dungeons and Dragons (something that WoW and just about every fantasy RPG borrows a lot from,) there is an idea of the Material Plane (referred to in this diagram as "Reality") having something called the Feywild that exists above it as a kind of whimsical and beautiful place that Fey magic comes from (Faeries and such.) But the flipside is something called the Shadowfell, which is a dark reflection of the world that is harsher and more barren and crueler. A nice snow-capped mountain might have a blackened, spiky volcano as its Shadowfell reflection.
I don't know for a fact that we've ever truly seen the Shadowlands in Warcraft, but I have a strong suspicion that we have. And it's all thanks to the Lich King.
Note that the Undead (not the playable race of Undead, who should really just be called Forsaken, but the Undead in the whole of the universe) are up there with Old Gods, Titans, and the Burning Legion as one of the top representatives of their respective Major Powers. And they are in direct opposition to the Wild Gods (though as noted with the Titans and the Old Gods, and actually the Naaru and Burning Legion for that matter, your opposite number is not necessarily your main antagonist,) which means that just as the Wild Gods have a special connection to the Emerald Dream, the Undead might have a special connection to the Shadowlands.
During Wrath of the Lich King, there are a handful of quests where you are sent into some sort of shadowy death-realm. The first one is right in the Death Knight starting experience, where you have your mount transformed into a Deathcharger. Later, Horde players at least go into the real of Shadows doing a quest for Koltira, during which for some reason only on one character that I did this, the Lich King arrived flying on a Frostwyrm to taunt me. Alliance players might be stepping into this realm when they try to investigate the rumors about the connections between the Vrykul and Humans in a village outside of Utgarde Keep. If those players make the mistake of approaching the Lich King within that realm (and dodging the Val'kyr who will insta-port you out,) you get a really interesting monologue prior to what you have to imagine is a very painful insta-kill.
The Lich King is kind of a fascinating figure in the Warcraft lore, because despite being very new - less than 30 years - and being created by a member of the Burning Legion - the Lich King seems like an inextricable part of the Universe.
I wonder if Kil'jaeden's actions toward Ner'zhul didn't actually create the entity that is now the Lich King, but really just created a vessel for the very essence of the Power of Death to fill.
How long have the Shadowlands existed?
Going into Legion, there's another interesting question: Where are the Halls of Valor and the Maw of Souls? I actually had a theory that the Halls of Valor might literally be in Skywall (given the "floating up in a bright and heavenly-looking place) while the Maw of Souls might be in the Abyssal Maw (there's Maw right in the name, plus it's where Kvaldir come from and they seemed to be working for Neptulon in Vashj'ir.)
It's possible that they're kind of their own realms, or perhaps they exist more explicitly in "The Light" and "Death" or something.
The point of this whole post is that we've been given the mother of all lore bombshells here. I suspect a lot of this will be elaborated on in the book, but it really gives us a profoundly different look at the whole Warcraft universe.