Blizzard feels that they made a mistake in assuming that Alliance players would enjoy the newly "neutral" Thrall, because he isn't a Horde character, he's a world character. That's part of the issue, although he doesn't understand the root cause of that particular sentiment, which stems from the treatment of other major lore figures who ostensibly went neutral, and how they compare to what happened with Thrall. That, however, is an issue that deserves its own post to lay out.
The majority of the backlash against Thrall in the recent expansion is, quite simply, because Thrall has been an incredibly poorly written character since the end of WCIII, and putting him in the brightest spotlight any previously defined lore character has ever stepped into put all those flaws on display.
Some of these flaws are incidental to concessions made by the story for gameplay purposes. One of the most dissonant experiences I had while exploring the Horde side of the game was taking my first Horde character, an undead warlock, over to Ogrimmar to get the quests for RFC. I embarked on a series of quest lines featuring Thrall trying to root out the warlocks from the midst of the city, and seemingly oblivious to the fact that he was employing one of those warlocks to do so. Despite the fact that from a lore perspective, Warlocks had nearly wiped out the orcish race, and condemned them to an existence of suffering, and as such were rightly banned from the Horde, people wanted to play as them, so in they went. These flaws are jarring, but they're ultimately a minor flaw, the pimple on the narrative that can be hidden with some discrete concealer, because the player character is but a single person in the narrative of the game, and not even a really important one anymore.
The flaws become much harder to conceal when they become part of major lore events. Thrall is not presented as a complex character. He's your standard feel good messiah stand in. He's wise, strong, capable, charismatic, fair, self sacrificing, and compassionate. There's not necessarily anything wrong with that. Not every character in a leading role has to be a morally ambigious Machiavellian schemer. It's OK to have the occasional character who's just a good guy. The problem is when you have the aforementioned good guy does something out of character, and no one calls him on his bullshit. At this point, you're not only undermining the character, you're undermining the characters who don't react, and the entirety of the story. If Aslan had given up Edmund in The Lion, The Witch, and The Wardrobe, and none of the Pevensies spoke out against it, then C.S. Lewis probably wouldn't hold his revered place in the pantheon of storytellers. The story worked because Aslan is the Jesus Allegory Lion, and he did what Jesus Allegory Lions do.
This is where Thrall falls short. He's the Jesus Allegory Orc, savior of his people, able to walk on water, and return from the dead. But he doesn't act the way his backstory and characterization have set him up. The driving influence in Thrall's backstory was his time spent as a slave and gladiator in Durnholde. So, when he fufills his destiny and forges a reborn Horde, does he outlaw slavery, as a just ruler would do? Nope, because one of the driving forces in King Wrynn's backstory is that he was captured and held as a slave by Rehgar Earthfury, and forced to fight in the gladiator pits in Dire Maul. Blackmoore was an uncharacteristic human, operating far from the oversight of authority, and was a drunk and morose man who plotted against his fellows and no one in the Alliance speaks well of. Perhaps, King Wrynn ran into a similar situation? Perhaps Thrall did take steps to stamp out slavery in the Horde, but its insidious nature persisted in the dark corners far from his steely gaze? This would be a bit of a stretch, but it's one that could have worked. But is it what happens? No. In the comic, Jaina convinces Varian to attend a peace summit at Theramore to attempt to build a peaceful relationship with Thrall and the Horde. Varian brings Valeera, a Blood Elf who escaped slavery with Varian, and his son Anduin. Thrall brings Garrosh, who from a chronological perspective, made the first of his asshat moments, and Reghar Earthfury, the orc who kept two of the members of the Alliance delegation as slaves against their will. Not only does he tolerate slavery within the Horde, he thinks it's a good idea to make the slavemaster a trusted adviser on the peace process. No one brought up how immensely hypocritical and bone headed this is, and yet people wonder why Varian doesn't like Thrall. It's completely out of character for someone coming from the backstory Thrall has, and it's completely out of character for everyone around him to not notice. Have the races of Azeroth simply not discovered the most basic of social prudence?
This leads into what becomes Thrall's most damning problem. He's a storyline singularity. A black hole that warps those around him such that even the most basic literary laws simply don't hold anymore.
The most egregious flaw in Cataclysm was the 4.2 quest chain which unveiled Fandral Staghelm as a traitor, and set the stage for the Aggra/Thrall love story that culminated with the probability of little green/brown babies in upcoming expansions. Staghelm was one of the best characterized and well written characters introduced in World of Warcraft. He's a seething mass of complexes and hatred and old wounds that refuse to heal. He was one of Malfurion's finest students of druidism, and after Malfurion went to travel the Emerald Dream, it was Staghelm who led the night elf forces against the Qiraji in the War of the Shifting Sands. The forces of the Old God C'thun poured out of Silithus, and the Night Elves alone stood against them. Fandral led the army in a brutal war in the deserts of Silithus and Tanaris. This conflict is the event that more than any other defines Staghelm. His only son, Valstaan, served with him in that harsh conflict. Staghelm recognized the threat that the Qiraji posed, not only to Night Elves, but to Azeroth itself, and he beseeched the dragonflights to aid him in his efforts to keep the Qiraji in check. The dragons deferred their entry into the conflict until the Qiraji had pushed all the way to the Caverns of Time. In one battle in the months before the entry of the dragons into the war, Valstaan was captured by a mighty Qiraji general. In front of Fandral's eyes, his only son, his pride and joy, was ripped in half, and left to die on the scorching sands of the desert.
One of the first short stories, and in my opinion, one of the best, that Blizzard made was in preperation for the AQ40 raid, and the opening of the Gates of Ahn'Qiraj. It showed the true measure of the toll that the war had exacted on Fandral.
Fandral looked down, his face twisting in contempt. "I want nothing to do with Silithus, the Qiraji and least of all, any damned dragons!" With that Fandral swung the enchanted object into the magical gates — where it splintered in a shower of fragments — and walked away.
"Would you shatter our bond for the sake of pride?" the dragon asked.
Fandral turned. "My son's soul will find no comfort in this hollow victory, dragon. I will have him back. Though it takes millennia, I will have my son back!" Fandral then strode past Shiromar...
... who could see him in her mind even now, as if it were only yesterday and not a thousand years past.
Clearly, this is a man who has lost much, and harbors much resentment over that loss.
The time preceding the games themselves, Fandral spent clashing with Tyrande over the path that the Night Elves should take. It was Fandral who pushed for the creation of Teldrassil , and who eschewed the blessings from the dragons which had empowered its predecessor. He was arrogant. After all, he had fought the War alone. He knew what a threat the Qiraji were, even before the Bronze Dragonflight could scry it from myriad visions of the future. There were those who would take advantage of his wounds, and his pride.
Xavius, the first Satyr, condemned to live as a tree by Malfurion for 10,000 years after the Sundering of the World, saw Staghelm's flaws as a means by which he could exact his revenge upon Malfurion. He used his magic to plant the notion into Staghelm's mind that through certain actions, he could bring his son Valstaan back to him. This involved using large quantities of Morrowgrain to poison Malfurion, trapping him in the corruption of the Emerald Nightmare, where Xavius could torment him at his leisure. It also involved grafting the tree that Xavius had become onto Teldrassil, exposing the Night Elves home to the corruption of one of Sargeras' oldest servants on Azeroth.
During the events of the novel Stormrage, Malfurion broke free, and confronted Staghelm at the demonic graft on Teldrassil, which Staghelm had come to believe was his son reincarnated. In order to stem the corruption of the tree, Malfurion was forced to destroy Xavius, and the graft. Staghelm, however, was forced to relive the death of his only son. Watching Malfurion kill Valstaan broke Staghelm, and he was sent away to the caves in which Illidan had been imprisoned for thousands of years.
From there, during Ragnaros' assault on Hyjal, Staghelm was intended to be moved to a more secure area in Moonglade. The green dragon Alysra was charged with moving him. She instead defected to Ragnaros, who offered Staghelm a chance to strike back against those that wronged him. Against the dragons, who refused to aid him until his son had died. Against Nordrassil, which had succeeded where his project, Teldrassil, had failed. Against Malfurion, who took his son from him a second time. It's a compelling tale about how the indifference of the forces of good can break a good man, and how a good man can be brought low, despite great physical power, through the exploitation of the weaknesses of the mind. It was one of the stories that I respected most in the Warcraft Mythos.
I've talked a lot about Staghelm in a post that is ostensibly about Thrall, I promise there's a reason for this. Staghelm had a lot of lore behind him. More than most other characters that were introduced in WoW proper. More than even some of the characters brought over from previous games. He was well written, and well developed. The player knows what he's been through, who he hates, and who he serves. It all led up to his becoming Ragnaros' Majordomo, and a raid boss in Firelands, and most importantly, it did so in a manner that made sense, in as much as a story with dragons and giant ant people fighting elves can make sense.
Which brings us to the quest Elemental Bonds. One of the centerpieces of the 4.2 patch, Rage of the Firelands, this quest served as Staghelm's coming out party as an actual villain. Malfurion and the four Dragon Aspects unite to attempt to restore the World Tree. Five of Staghelm's most hated enemies, and Staghelm's hatred has brewed for millennia. Malfurion would go on to lead an assault into the heart of the Firelands, and eventually permanently destroys Ragnaros, Staghelm's new master. Not only does Staghelm hate Malfurion, but Malfurion is also a present threat to Staghelm's new employer. It's a two for one deal for vengeance, always a good move. So, with Staghelm's history, circumstance, and new found power, he crashes the ritual. His five most hated enemies, caught distracted and vulnerable. So what does he do? He attacks Thrall.
A story that Blizzard has been brewing for seven years in real life, and spanning thousands of years in game, and they wreck the ending just to show how awesome Thrall is. That's how destructive Thrall has become to the fabric of the narrative of the game. He's creeping into story arcs that don't involve him, and ruining the endings. Characters are taking complete 180s to bow to the amazing Go'el.
There's a term for characters like this. They're called Mary Sues. They're generally reviled for being an extremely disruptive force in narratives. A lot of times, they serve as author inserts. A way for the writer to live out his story vicariously through the character. This isn't always the case, but they manifest as characters that lack depth. While this is OK for peripheral characters who don't impact the story greatly, when it's the main character, by dint of the amount of time the reader or player spends dealing with the character, they have to be believable. Familiarity breeds contempt. The more time you spend with a character the more the little flaws in the story irk you. It was fine back in Vanilla when Green Jesus could sit on his Throne in Ogrimmar and generally not bother anyone because the story at the time was about the player-character, and as such, it welded itself to the person playing that character quite well. It's the classic silent protagonist that worked so well for Link, Mario, and Crono in earlier games.
In the expansions, Blizzard moved away from the notion of the story really being about the player-character. They had to, because it was a persistent world, and having Bolvar greet me as the one who unmasked Onyxia when I did no such thing in game would be a jarring flaw in the game. As such, the players went from the driving force of events in game, to a more abstract ideal. They're "the adventurers". They're always there, but they're never the reason why the big events happen, because if your character didn't do a particular quest, or dungeon, or raid, some else did, and to keep the game on a single narrative track requires that those loose ends be tied up somehow. Blizzard opted to make NPCs drive the story. Illidan was defeated by Maive and Akama, with an assist from "the adventurers", Kil'Jaeden was banished by Kalecgos and Anveena, with an assist from "the adventurers". It continued into Lich King with Tirion, and Blizzard, in an effort to create a cohesive narrative, went as far as to retcon the Vanilla raids. Varian killed Onyxia, and Darion Mograine stormed Naxx. The players have been relegated to extras in the story that used to be about them.
This isn't a bad thing, per se. The problems have come to a head with Thrall in Cataclysm because Thrall is so poorly written. Thrall has been spoiled. The weight of the failures of the writers have ruined him as a character, and he is beyond redemption. He's been woven so tightly into the fabric of the game that a discrete retcon is impossible. Trying to continue Thrall's story will have ruinous consequences on the narrative, and trying to retcon the mess they've already published would be the most disruptive retcon in the history of the Warcraft Mythos. The time has come to give Thrall a good Wesley-ing. Well, the time came a while ago, but late is better than never. Retire him to Nagrand to raise his little green/brown kids. Kill him off. I don't care. But stop inflicting him on the story.