I started this post about a month ago, but some other issues cropped up, and I've just now gotten around to finishing it, my apologies if some of it's out of date. Also, looking back over it, there's spoilers for a lot of things beyond WoW here. Be warned.
A big complaint about Cataclysm was that Deathwing wasn't a very compelling villain, especially in comparison to the titular Lich King of the previous expansion. There's a lot of reasons for this, but I think the most prominent is one that WoW shared with another major video game storytelling failure, Mass Effect 3. Both Blizzard and Bioware made the mistake of undercutting their primary antagonist.
The undercutting of a villain occurs for several reasons. Sometimes it occurs because the writer wants to foreshadow a future plot arc, and takes it too far. Other times the writer tries to add additional complexity to the plot, and fumbles it.
WoW runs multiple antagonist storylines, and as such, it gets a little difficult to track which one is active at any given time. You've got the Burning Legion under Kil'Jaeden, Sargeras and his portion of the Burning Legion, which might be at odds with Kil'Jaeden's crew, the Old Gods trying to corrupt everything, the Titans playing a game so large and vast that the entire world might get crushed with all the concern that an wrecking crew has for the roaches in a condemned building, The Scourge, many of whom are now independent, the war between the factions, The Black Dragonflight and their insane patriarch, the Troll Empires, and many more lesser foes that I'm leaving out. Each of these groups vie for the right to be the current target of the player's ire. Mass Effect on the other hand, really only has one enemy, the Reapers. Sure, there are batarian slavers, krogan warmongers, and douchebag turians to deal with, but hey, when the Reapers liquidate the human race, they all seem like small potatoes.
Both the Old Gods of WoW and the Reapers of Mass Effect share the same common root for their origins. Both of them are HP Lovecraft ripoffs. Blizzard's incarnations are blatantly and unashamedly so, while Bioware tries to mitigate it by stealing heavily from The Armageddon Inheritance, very heavily.
HP Lovecraft is a popular starting point for a lot of fiction, not because of his writing style, which was so chock full of gilded passages that Hemmingway would have cried if he read it. What makes his work compelling is that he took the dark, dismal world of victorian era authors like Poe and Melville and cranked it to the extreme. He created a world of monstrosities that aren't evil, they're simply so far beyond humanity that our entire race might be wiped out by Azathoth's burp. It's not a mater of black and white morality stories, it's mauve and cyan morality, and the decisions to be made will drive men mad.
What makes this fertile fields for the more action oriented stories that you find in WoW, Mass Effect, or works like The Atrocity Archive, is because it makes creating a hero extremely simple. He just has to not go insane. By simply being able to stand against the monstrosities out there, he's already a better man than most heroes.
A work that draws upon Lovecraftian principles tends to have several key elements that get drawn together. The disbelief of the larger populace, if they confront this threat, they have to acknowledge its existence, and their pitiful minds lack the fortitude to do so. This plays out in both WoW and Mass Effect. The Dragonflights refused to acknowledge the threats posed by C'thun's forces in Silithus until the Qiraj almost kicked down the doors to Norzdormu's house, leaving it to Fandral Staghelm to hold the line as best as he could with his followers. The citadel council repeatedly ignores all evidence of the Reapers, up to and including a Reaper attacking the Citadel, leaving it to Commander Shepard to hold the line as best as he could with his followers.
Another key aspect is the Elder Things themselves, a creature vast an incomprehensible, that's liable to drive you insane if it doesn't kill you outright. The Old Gods and Reapers fill this role in their respective universes. Tentacles are often used to represent the alien nature of the Elder Things, a common feature of every Old God and Reaper seen.
Another common element is the Deep One hybrid from The Shadow Over Insmouth that was created to serve Dagon. Wow is rife with Faceless Ones, Elemental Acendents, and the Quiraj and Nerubians, all of whom were once other races, but were twisted to serve the needs of the Old Gods. Mass Effect also jumps in with both feet here, with the implications of the possibility that many of the races that the Reapers wiped out were twisted, the Protheans being turned into the Collectors was made explicit, and it's likely that the Keepers in the Citadel were another one of the races the Reapers wiped out. In a plot line that I thought was masterful, but was ultimately discarded in order to keep the story aligned to that of The Armageddon Inheritance, was the implication at the end of Mass Effect 2 that the Reapers themselves were hybrids. Techno-organic hybrids that required compatible sentient races to reproduce themselves. The notion that your whole race amounts to sperm for this alien behemoth, and wiping out your civilization was just foreplay for them drives home the notion of just how small you are compared to them. I thought that that was an incredible idea, and it was one that explained the whole notion of the cycle, one of the biggest questions in the Mass Effect universe.
Now that I've shown you where these two stories were coming from, let's examine what went wrong.
In Mass Effect 3, things are going swimmingly until the very end. It was quite possibly the most disappointing ending to a game I've ever seen. After you raise an army to come back to Earth and build a superweapon called the crucible that gives you a chance to take out the Reapers, pretty much everyone gives up everything in order to give Shepard the chance to take out the reapers. He activates the Crucible, and this is where things go wrong. Shepard is confronted by an AI represented by a five year old kid who explains that he controls the Reapers, and if you can convince him to back off, he'll take his pet techno-eldritch abominations and go home. This happened in Mass Effect, because it happened in The Armageddon Inheritance. That story ends with the ancient army that wipes out all sentient life every 50,000 years as being directed by an AI, which explains a lot of the issues set up in earlier segments, such as the Achuultani's inconsistent technology levels with regards to Gravitonics. It sets the stage for the reveal that the human's Superweapon, was in fact an AI that through 50,000 years of maintenance free existence it had transcended its core programming and decided to fight for humanity. Mass Effect had no such reveal planned, and had no prior set up to justify it. It just suddenly went from the Reapers are the ultimate threat in the universe, to the Reapers are the toys of a petulant child. It completely undercut their credibility, and cheapened everything the player had accomplished in three games. If they had simply carried on with the amazing story they had assembled up until that point, it would have been a slam dunk, but slavish adherence to source material without understanding the underlying mechanics creates problems.
Cataclysm, on the other hand, set up the Old Gods all through Cataclysm. The Old Gods were explicit throughout the the raiding tiers. They were behind Cho'gall, and two of the major end of raid bosses were Elemental Lords, Ragnaros and Al'Akir, who were explicitly powerful servants of the Old Gods. There were giant gaping maws in Twilight Highlands, the capstone zone for the expansion, and there were two Faceless Ones as bosses in the final raid of the tier. They made it explicitly clear that Deathwing was serving the Old Gods.
However, Cataclysm was supposed to be Deathwing's expansion. Instead, they sold out the entire expansion trying to set up future story arcs. Southshore, which was originally going to be destroyed by Deathwing's Cataclysm, was instead wiped out by the Horde, as part of the poorly thought out campaign to ramp up faction tensions in MoP. Go through the zones released for Cataclysm, and it's the same thing through and through. Vas'jir is all about the Old Gods and the Naga. Hyjal has a token visit from Deathwing, who is promptly never mentioned again, and then it's all Ragnaros all the time. Uldum has the Black Dragonflight's influence at times, but it gets drowned in Nazi comic relief quests. In the Twilight Highlands, aside from a few side quests, it's mostly about the Horde's invasion, and Cho'gall and his cult serving the Old Gods. Deepholme is the only zone that's focused on the events that the expansion is built around, and even that's only tangential.
In raid content it's even more palpable. Tot4W is about Al'Akir. Bastion of Twilight is about Cho'gall, while Sinestra might be the final boss, and one of the best of the few sections of the expansion actually dedicated to Deathwing, the overwhelming majority of raiders downed Cho'gall, and never even saw Sinestra. BWD was a magnificent raid, but didn't have the slightest connection to the outside world. Firelands didn't have a single mention of Deathwing, and even in Dragon Soul, Deathwing's apex raid, there's two bosses devoted to the reminding the players that the Old Gods are really in charge. That's like having two Burning Legion bosses in ICC.
Contrast that to Wrath of the Lich King, where the Lich King and his Scourge was a palpable pressence in every zone. Even in the lighthearted Sholozar Basin, they made sure the Scourge showed up in a manner designed to create the most emotional impact possible. The Lich King was a palpable presence in every raid tier. Naxxramas was his advance guard, ToC was the preparations for the assault, and ICC was his fortress. Even in Ulduar, the final encounter provided keen insights into the story of the expansion.
That's not to say that you can't foreshadow future plans, that was something else that they did well in Wrath. The Nexus War introduced the Aspects into the game a full expansion before their time in the spotlight, and the fights with Sartharion, Malygos, and Halion showed the plots brewing for Deathwing well in advance of Cataclysm. Hell, the Yogg-Saron encounter didn't just give us great insight into the path of the Lich King, it also showed us glimpses of Deathwing's decent into madness for Cataclysm, and the roots of the conflict between Orcs and Humans in Mists of Pandaria. But the story of the Lich King was always front and center. There was never any doubt or ambiguity about that.
The fundamental problem with Deathwing's Cataclysm is that Blizzard spent so much time foreshadowing their future stories, they forgot to actually tell Deathwing's story. They should have definitively linked Deathing to Nefarian's return. They should have explored Deathwing's origins, his actions, and his relations with his "siblings" much closer than they did. When your story is nothing but foreshadowing for the next story, then it's not a story, it's a prologue, and no one gives a damn about the villain in the prologue.