I stopped playing at the end of Mists of Pandaria because of my dissatisfaction with the storyline -- but since then I've spent a bit of time thinking about what went wrong. (I keep track of the story in the vain hope that it'll improve at some point.) At this point, I think they've built up a lot of inertia behind bad story practices, and don't know how to stop.
Originally, in vanilla, there were things to like and dislike about both factions. The Horde were all coming from a bad place -- whether it was as minor as the Tauren being rootless and never having a chance to build a homeland due to being hunted by the centaurs and quillboar, or as major as the orcs having been part of a demon-driven army, or the Forsaken having been killed and used as a mindless zombie horde. But they were all coming out of dark places, and trying to be better than that. Some were more committed to this than others -- the Tauren have always been pretty all-around good, while the Forsaken (or at least their leaders) were always pretty much cartoonishly bad -- but at least parts of each race were trying. There seemed to be an indication that the main struggle for the Horde would be whether they could rise above their past and become something greater, or get pulled down back into the dark.
Meanwhile, the Alliance races were starting off from places of light and prosperity -- but there were strong indications of internal weaknesses and failings in each race, things that could drag them down. The humans had selfish, corrupt, racist nobles, and an underclass of people who felt wronged by them. The dwarves had almost a race war going -- Magni ordered the killing of the Dark Iron king because he couldn't believe his daughter would willingly marry 'one of them.' The Night Elves had a strong xenophobic, isolationist streak to them, with even a bit of supremacy. And, of course, the gnomes had lost their home due to the political ambitions of one of their own. The implicit plotline for the Alliance was working to preserve what they had, and facing their own dark sides before they could be dragged down by them.
The trouble is, in those early days Blizzard didn't have any experience with how to tell a story in an MMO. (Not many people did, to be fair -- it was still a pretty young field.) And they had no idea how long the game would last. So they 'resolved' the whole human plotline in the first raid, chalking it all up to Onyxia's influence, and presuming that killing her would solve everything. Other plots were given similarly trivial treatment. And then everything... stagnated. The surface impressions of the Alliance being a shiny happy place and the Horde being a rough-n-tough brutal place survived, but the undercurrents that gave each of them depth were lost. In particular, the Horde was made to embrace the dark places they were coming out of. The orcs remained a brutal demon-influenced militarist culture ruled by an unquestionable military dictator. The Forsaken kept on hating having been made into emotionally-deadened zombies and yet decided their 'race' couldn't be allowed to die out. And so on.
(I also have to quibble with something here: Sylvanas was never a 'morally grey' character, at least not since her undeath. No, not even back in Warcraft III. In her first missions, back in The Frozen Throne, she's malevolent, brutal, and very eager to betray her allies before they get a chance to betray her. There's a common narrative that the Forsaken were rejected by humans who were scared of the undead. But that never happened, at least not before any more recent retcons I may not be aware of. The first humans the Forsaken came across were Garithos's army. Now, Garithos was probably the worst racist the Alliance had to offer at the time. And yet even he was no worse than somewhat skeptical and surly about her and her army. She promised him that she just wanted revenge on the demons who had made her this way, and offered to help him retake the capital city, then leave peacefully. And he accepted this, even putting himself and his men under her command. But as she confided to Varimathras immediately after making that offer, she planned to betray him from the first. And she did, slaughtering him and his army after they'd gotten her what she wanted. The Forsaken were never victims of the humans. Of the Lich King, yes. But from the beginning, when it comes to humans, they've always been the victimizers, and Sylvanas first among them.)
Through at least vanilla and Burning Crusade, and arguably much of Wrath of the Lich King, there was very little in the way of actual development for the Horde and Alliance as a whole -- just set pieces that reverted things back to the status quo at the end. That was the nature of the storytelling at the time -- they didn't have any methods in place for really doing a storyline that lasted longer than a single patch. And so the status quo got firmly established, and cemented as 'what should be' to both the developers and the players alike. The Horde must always be a military dictatorship that uses force to get what it wants. The Forsaken must be a zombie army that does horrific experiments on the living. Stormwind must be a beautiful fairy-tale kingdom where the major conflicts have all been resolved. And so on. When they eventually got more daring and started doing storylines that changed things up, this was the baseline, and the only thing the developers could do to it that players would accept was turning things up to 11. Make the orcs more aggressive and brutal, make the Forsaken not just an aggressive zombie army but a rapidly-expanding one that slaughters entire towns, make the humans a nation of paladins whose only major conflict is 'other races don't respect our king enough, and we have to prove them wrong.' Of course, this got eventual pushback, too -- but attempts to take things in other directions would get even more pushback.
The point where it became clear to me that there was no will for actual meaningful change and growth was the end of Mists of Pandaria. The Horde had just been given a strong reminder that the position of Warchief, a military dictator who the whole Horde swore personal allegiance to and who could only be dethroned by someone capable of defeating them in a duel, was bound to be abused when the wrong strongman got into power. This could have been the impetus to change things. Abolish the position of Warchief, and instead set up a council of elders or something like that, where members were picked for wisdom instead of might. Make Warlords of Draenor's Horde half be about delving into the past of the orcs, finding out what their traditions were like before being distorted by the demons, letting them learn from the past but not be bound by it as they grew for the future.
Instead, they decided 'the only thing wrong with the Horde today is that the wrong person was on the throne.' Get rid of Garrosh and everything is fine forever. Back to the status quo that nobody can imagine departing from. And, well, we can see how that worked out. I knew upon seeing that cutscene that it would only be a matter of time before Sylvanas or another expansionist orc or some other aggressive dictator was Warchief -- and when that day came, the Alliance would never make good on Varian's promise to 'end' them, since that would be destroying a player faction. As someone who played primarily for the story, I knew there was nothing for me to look forward to from there.
I'm sympathetic with the people who wanted to play the original promised storyline of the Horde -- the 'dark,' 'monstrous' races struggling to rise above their pasts and build something new and lasting. But Blizzard's never really delivered on that, and with nearly fifteen years of inertia keeping the plot barreling down the same direction, and them struggling to hold on to what loyalist players remain long after the game's peak, things aren't going to change.