With Betrayer's Rise now easily accessible to anyone through LFR, I think we can take the spoiler tags off of what happens at the end of it.
I know I've heard some people hoping that Gul'dan (and to be clear, this is Gul'dan B we're talking about) would survive to plague us some more, but this doppelganger Gul'dan is now just as dead as his main-universe counterpart, dying actually across a narrow strait from each other.
What we have accomplished in finishing the Nighthold raid is actually quite a few things. First, with the defeat of Elisande, the Dusk Lily rebellion has succeeded. While for gameplay reasons Suramar's always going to have hostile Duskwatch and demons, lore-wise Thalyssra and her followers have taken the city.
With the removal of the Eye of Aman'thul from the city, the Nightwell will die. The good news is that with the Arcan'dor, the Nightborne should be able to escape the addiction that would have turned them into withered - something that the Blood Elves actually never pulled off (though given that the Sunwell is now half Arcane and half Holy, they're in pretty good shape.)
This does probably mean that we won't be seeing much of the Nightfallen crew anymore - they have to rebuild their society amid a continuing Legion invasion. I liked these characters, and would not mind seeing them again in the future, but I'm skeptical that we will.
Gul'dan is dead. Frankly I expected him to be the final boss of Warlords of Draenor, but I think it's actually kind of interesting how they used him to bridge the gap between expansions (much like Garrosh did previously.) And while I was never a huge Varian fan, getting justice for what happened to him on the Broken Shore felt pretty good.
But let's also talk about the biggest thing to come out of this:
Gul'dan was trying to use the Eye of Aman'thul to turn Illidan's corpse into a vessel for Sargeras. He tore a rift into a very red part of the Twisting Nether, but upon his defeat, the rift collapsed. And in doing so, the crystal that held Illidan was broken, and with his body free, his soul was able to return to it.
Illidan does actually steal our kill, incinerating Gul'dan from the inside just as the latter had done to Varian. And he crushes his skull for good measure (actually, thanks, Illidan. The last time Gul'dan's skull was left lying around it caused all sorts of problems.)
The big point, though, is Illidan is back from the dead. And as Demon Hunters would know from their campaign, while he has been dead, he has still been able to see what's happening, and it looks like he's ready to get the old plan to destroy the Legion back on track.
It's actually pretty exciting to see a cutscene after a first-tier raid boss. I'm hoping that Blizzard will start doing this for all the major raids, given how since Wrath it has only been for expansion-bosses.
We have to wait for 7.2, but at this point we have Illidan back, Suramar has been liberated, and we now have all five Pillars of Creation. Now we're heading to the Broken Shore to take the Tomb of Sargeras and use those pillars like we planned.
And nothing could possibly go wrong, right?