With the Legion Beta live (fingers crossed for an invite!) players have been able to access some of the quests in which one acquires one's artifact weapons. I'm pleased to report (thanks to other websites like MMO-Champion) that these quests look pretty interesting, with special challenges that use your spec's skills. I don't know how challenging these will be (I think they'll have to be relatively simple, as this is the very gateway into the expansion) but there's also the added benefit that it will send players off in 36 different directions when the expansion starts. Not only will the "adventure mode" style of questing prevent bottlenecks, but everyone's going to be scattered to the four winds and off in twelve different class halls (not to mention new Demon Hunters, who will be doing their own starting experience before they get to these quests.)
Anyway, I've seen videos of the Frost Death Knight and Fire Mage quests, which both involve a trip to Icecrown Citadel. In them, we get some insight into what is going on with the Scourge and Bolvar, the new Lich King.
Spoilers beyond the cut:
The Fire Mage sword, Felo'melorn (which looks like a two-handed sword, but also comes with an off-hand item,) was the blade of the Sunstrider dynasty. The sword literally means "Flamestrike" in Thalassian, and is unsurprisingly associated with Fire Mages. After the ill-fated march on Icecrown by the Illidari, Kael'thas lost this sword battling against the Scourge.
One of his distant relatives - from a branch so far removed that, while they still have the name, they have no true claim on the throne - tried to acquire the sword, but she became trapped within Icecrown Citadel.
It's clear from the dialogue with Bolvar that the new Lich King may not be evil, but he's sure not nice. The Lich King reads your mind right as you enter his fortress/prison and flat-out tells you that he is not going to lift a metaphorical finger to help you against the undead he has trapped within (obviously his literal fingers are still frozen inside the Frozen Throne, so he won't lift those either.)
Still, once you acquire the sword, he recognizes you as worthy of wielding it, and sends you back to Dalaran with his approval.
Not surprisingly, the Frost Death Knight quest is much more involved with Scourge lore. With Bolvar having taken over after Arthas' death, the relationship between the Scourge and the Ebon Blade has changed. The Knights of Acherus once again heed the call of the Lich King, and he summons you to arm you with the most tremendous weapon available to him.
The Ashbringer shattered Frostmourne when the Arthas was defeated, and most of the souls trapped within the blade were freed. But as Bolvar reveals, there were still some that remained. After defeating some of the trapped souls, including Baelgun and some other important figures from Arthas' life, Bolvar sends you into the hilt to deal with the last remnant.
Inside, Arthas and Ner'zhul remain within the sword. You need to defeat Arthas while Ner'zhul sends waves of the undead against you. This is pretty surprising, as previously we had believed that Ner'zhul had been fully consumed by Arthas. However, now the old Orc Shaman, and the very first Lich King, seems to be trying to use Arthas' soul to escape from the blade.
What could this mean? Was Ner'zhul never really gone? Had he been pulling the strings? He did, after all, refer to himself as a former shaman in Howling Fjord.
When Admiral Taylor's garrison was massacred, the necromancer behind the plot referred to a "dark master." Might that have actually been Ner'zhul? Not the at-that-point-still-living Ner'zhul of the alternate universe, but rather the Ner'zhul who had been the first Lich King?
Upon purging the blades (or perhaps merely subduing the souls within,) the Lich King summons you to his throne, and there he declares you his champion, the Hand of the Lich King. That's right, Death Knights in Legion and onward will once again fight in the name of the Lich King and the Scourge - albeit a version of them that seeks to defend the world, rather than kill everyone on it.
It's really amazing that they're putting in this much effort for each quest specific not only to a class, but a spec. You'll be able to spend your first hour or two of the expansion doing really cool spec-specific content if you decide to get the artifacts for each spec.
While I'm sure that after getting your artifact, things will narrow down a bit, and perhaps we'll just have class-specific stuff instead of spec-specific, if they can keep this stuff up I will be overjoyed.