While we still don't have a really solid idea of how all their mechanics will play out, we got a couple of quick little details on Demon Hunters and how they'll play or be worked into the game.
First off, apparently they actually wanted Demon Hunters as early as Vanilla, but they also tried to work them into Burning Crusade. If you remember how class balance was during either of those time periods, you'll understand why Blizzard was hesitant to add another class to the pile. They would have obviously fit quite well in Burning Crusade's theme, but they were not yet ready (and I'm actually happy about this, as I think it could have delayed the introduction of the Death Knight. As excited as I am for Demon Hunters, the Death Knight is probably my favorite class in terms of concept and backstory - and clearly I like the mechanics, as my DK is my primary alt.)
Demon Hunters will use a resource called Fury, which they said will work a lot like Rage - meaning that you'll build it up and then spend it, but the way in which you do so will differ from the Warrior enough to make it feel different. I imagine it'll be somewhat like the difference between Energy and Focus - very similar in concept, but very different in feel. I'd look to current Demonology Warlocks' Demonic Fury as probably the inspiration for this, which suggests that you'll build up and spend Fury slower than Rage, giving you a kind of build-and-spend rhythm to the class.
They also mentioned that they might open up Demon Hunters to other races in expansions after Legion. While I'm very sad that I won't be able to do a Worgen Demon Hunter, I also can see that this is a very art-asset-heavy class - possibly even more than a Druid, because a Druid's features in humanoid form don't really make a difference in their various shapeshift forms (outside of hair/fur color.) Demon Hunters, however, look like they'll still look more like their humanoid selves while Metamorphized, which means that limiting it to two (fairly similar) races makes a lot of sense.
Of course, I wouldn't hold my breath for them to open it up to other races. This will certainly happen in 8.0 at the very earliest, and even then, they'd have to get around the whole idea that Demon Hunters begin as members of the Illidari, which was famously an Elf-only organization (I guess you might count the Fel Orcs and Ashtongue Broken Draenei, but they didn't really seem to be in the inner circle, and you never saw any of them training to be Demon Hunters.)
Given the setting of that starting experience in the past, it would be pretty hard to really work in any other races into the starting experience. Frankly, if it had been my design, I'd have had the playable Demon Hunters be part of a new organization founded in current times, much like the Knights of the Ebon Blade were during Wrath, to allow for more races. So whether they'll have to come up with a radical tweak to the quests or just kind of smash the lore should it open up to other races (Worgen, please,) they've made it pretty hard to do so should they change their minds.
Can't wait for the Beta (to which I will hopefully get an invite) to try these dudes out.