By my count, Mists' Y'Shaarj-influenced Siege of Orgrimmar will make three of the five releases (including Vanilla) that was overshadowed with an Old God-esque ending: Vanilla, Cataclysm, and Mists. You could also make an argument that the Lich King, while tracing a lineage back to the Burning Legion, is also heavily influenced by Yogg-Saron*; after all, what exactly does Arthas use as a metal for his devices and buildings but Saronite, the blood of Yogg-Saron itself. And don't forget the Old God influenced quest chain in Icecrown where the ghostly child teaches you about Arthas and his heart; there's a reason why we found Arthas' heart in that strange area in the first place.
What makes the Y'Shaarj tie-in so disappointing to me was that the entire concept of the Sha was so new and interesting that it seems a shame that Blizzard couldn't let it stand on it's own. Just like how the Mogu had to have the help of the Zandalari Trolls, the underlying cause of the Sha just had to be the Old Gods.
I suppose you could say that Blizzard has an addiction to conspiracies. The popular uprising in Westfall characterized by the Defias simply couldn't stand on it's own, it had to tie in to the Twilight's Hammer somehow. The overeager and blind self righteousness of the Scarlet Crusade couldn't stand on its own as Garithos' racism and arrogance did in Warcraft III, it had to be tied back into the Dreadlords and the Legion.
Maybe that works for a while in a fantasy world, but the problem is that in the real world a lot of stuff just happens. There is no dark conspiracy behind a lot of criminal activity; a lot of it is a crime of opportunity (or passion). If there is a plot involved, it is very localized (one spouse hiring a hitman to take out the other spouse, for example). Sure, there's organized crime, but you can't blame everything on the mob. If there's a drug turf war, it tends to unfold organically, not manipulated by some master puppeteer in the shadows.
Fantasy lends itself well to that evil overlord, the shadows in the dark controlling our lives. But when you dip into that same well too often, it starts to feel forced and loses its punch. The most unique thing about Mists was the Sha, but it turned out to be just more Old God trickery, lessening the impact that it could have had. When all questlines lead to the same ending, all that's left for variety is the kill ten rats.
Perhaps that is why I've seen a lot of griping lately that WoW's high point was Wrath. Wrath had one raid that was the culmination of a long questline that had absolutely nothing to do with the "Let's Get Arthas!" movement: Ulduar. Was it Old God related? Yes. Was it a big, tough raid? Yes. Did it advance the Arthas story? No. Not one bit.
Ulduar was part of a giant three pronged fork in the entire Northrend questline --Arthas and Malygos being the other two-- and it demonstrated that a story didn't have to be part of the main part of the expac to be meaningful. Blizzard has gotten away from that with Cataclysm and Mists, and to add insult to injury they end up reusing the same old same Old (Gods) as a crutch.
I guess that we're going to be treated to yet another dose of Old Gods fairly soon, assuming that The Dark Below turns out to be the name of the next WoW expac. After all, what tends to inhabit the dark places of the world but Twilight Hammer and their ilk?
*Certainly in hindsight people still talk about Ulduar as the high point of WoW raiding, and I have to admit I liked Storm Peaks much more than Icecrown.