I don't typically talk about lore from a "down on the ground" perspective. I generally try to focus on broader concerns and major characters - the nature of the Titans, the motivations of the Infinite Dragonflight, the interaction between Fel magic and the Old Gods. But with a second hero class, I thought I'd talk a little bit about our upcoming Demon Hunter characters.
Personally, I'm going to be confined to my laptop the day that the class comes out, and I might simply wait until I get back from the airport, probably jet lagged and feeling like crap, to create my own DH (actually, DHs, as I fully plan on making a Horde Illidari in addition to the Alliance one.)
Mechanically, hero class has come to mean a class that starts at a higher level and for that reason has its own unique starting experience. For almost eight years, Death Knights have been the only hero class, but we're getting a second one next week.
But flavorfully, this starting experience actually emphasizes the way that members of a hero class are really defined by that class. Becoming a Warlock, Paladin, or Hunter is a life choice, to be sure. But ultimately all of the non-hero classes are essentially a set of skills. It might be very difficult for a lifelong Warlock to seek redemption and wield the Light as a Paladin, but it's not impossible.
Likewise, members of the non-hero classes probably relate perfectly well to those of other classes. A dwarf Hunter and a dwarf Warrior could easily have grown up together, even served together in the name of King Magni back in the day. That Hunter would probably feel a much stronger connection to her warrior friend than some Night Elf Sentinel who uses the same techniques she does. Granted, she might get to know and respect that Sentinel after the Alliance brought the Night Elves into the fold, but there is not, inherently, that sense of kinship there.
For hero classes, though?
I think the clearest example is the friendship between Thassarian and Koltira. Now, Thassarian is actually the guy who killed Koltira in the first place, but as members of the Scourge, they actually managed to set aside the old racial distrust (though at the time Thassarian killed Koltira, humans and High Elves were actually allies.) As a Death Knight, one really feels more attached to Acherus and the Ebon Blade than one's original homeland. Your fellow Knights know the cruel addiction you suffer while your countrymen mostly treat you with distrust and disdain.
This effect is deepened with Demon Hunters. The Lich King was pretty equal-opportunity in creating his Death Knights - the Pandaren are the only race that can't be DKs and that's just because Blizzard couldn't figure out a decent workaround for the timeline the way they had for Worgen and Goblins. The Demon Hunters are very specifically elves - Night Elves for the Alliance and Blood Elves for the Horde.
So you sort of have the racial unity built in to the class identity, unless they eventually (in a later expansion, if ever) loosen the restrictions. The Blood Elves are of course distant cousins (or, given the longevity of the Night Elves, nieces and nephews) of the Night Elves.
But at the same time the history experienced by the Illidari is a very different one than the other playable elves.
The first thing to consider is that everything from the end of the Black Temple Raid to the beginning of Legion - so Sunwell Plateau, all of Wrath, the Cataclysm, the war between the Alliance and Horde, and the whole Draenor situation - all of those events are things that your Demon Hunter has totally missed. They'll be stepping into a world that they left under threat of the Scourge and in which Deathwing was a lurking threat and find one that is completely transformed.
But that's not even all of it.
Illidan was freed during the Third War. We don't know for a fact when he first started training apprentices - one would assume that other Demon Hunters from that era would have been killed or imprisoned along with him, so we can probably assume that notable Demon Hunters like Feronas Sindweller, Altruis the Sufferer, and Loramus Thalipedes were probably trained in the period following the Third War, when Illidan was banished from northern Kalimdor but no longer imprisoned.
Given that the Night Elves only joined the Alliance in the period following the Third War and Illidan's banishment, that means that any Night Elf Demon Hunter would only be joining the Alliance now - they last experienced life in their society during the highly bifurcated period before the war. If your DH is a woman, she was probably a Sentinel or a Priestess of the Moon (or possibly a Warden who became intrigued with her most high-profile prisoner's techniques,) and if you have a male character, he was probably a druid sleeping in the barrows (though there are some non-druid male Night Elves like Jarod Shadowsong.)
Blood Elf Demon Hunters would have joined Illidan when Kael'thas went to Outland, escaping from the oppression of Garrithos (the human commander of the remnant of Lordaeron's forces.) So when they left Azeroth, they were seeking an escape from the Scourge and being used as cannon fodder by the Alliance. Illidan had provided the Blood Elves with a source of magic that sated their addiction following the corruption of the Sunwell.
The Illidari were most likely well aware that Kael'thas had betrayed them. Most of the Blood Elves who came to Outland followed him in his disastrous alliance with the Burning Legion. Indeed, given the Havoc Demon Hunter artifact quests, it looks like some of the Demon Hunters even followed him. But presumably your loyalty was to Illidan - your mentor and trainer who had provided salvation to your people.
Returning to Azeroth, you'd be aware that your people back home joined the Horde, though you've never lived under the Horde personally. The last time you saw your homeland, it had been utterly devastated by the Scourge, and so finding out that the Lich King had been defeated and that the Sunwell had been reignited with Holy magic might make you seriously question whether you had been right to follow Kael'thas in the first place.
Something that is apparently in the Illidan novel actually explains more about the nature of Demon Hunters, though, and this is where it starts to become clear that DHs have undergone an experience that is just as transformative and irreversible as that of Death Knights.
Illidan's recruits into the Demon Hunter class are all people who have lost a loved one or loved ones to a demon. Ironically, Illidan doesn't really fit this description, but it's part of his recruitment requirements. Probably the greatest trial in training to be a Demon Hunter is when the Demon Hunter must summon the demon who killed his or her loved one. The novice must slay the demon and then consume its heart, creating a deep connection between the demon and the hunter. The demon becomes a part of the Demon Hunter, a constant reminder of that painful loss but also a source of incredible power. The Demon Hunter's discipline is all that prevents the demonic presence within from taking over.
In a sense, a Demon Hunter has the opposite problem of a Death Knight. A Death Knight is numb, feeling only an endless hunger to inflict pain. A Demon Hunter is constantly assaulted with pain and rage, feeling it all, and indeed seeing all the demonic evil in the universe. Demon Hunters apparently do not gouge out their eyes as some careful ritual - it's more of a side-effect of being able to see through the eyes of the demon they have imprisoned within themselves.
Demon Hunters are transformed into half-demons themselves - their souls are bound to the Twisting Nether, like that of a demon, which allows them to return to life (granted, in-game they'll probably find Spirit Healers - which we now know are benevolent rogue Val'kyr - in the Shadowlands - which I theorize is where we go when we're in ghost form.) In a sense, they aren't really Night Elves or Blood Elves anymore.
But despite their demonic nature - or more likely, because of it - they are fully committed to fighting against the Burning Legion and the other evils that threaten Azeroth. Unlike Death Knights, they chose this path willingly, but either way, they have made a commitment that even death will not release them from.