Originally Posted by
Flurryfang
What happened was that they ran out of content, that built on Warcraft 3 and had to create their own lore from "scratch". Warcraft 3 did a really good job with establishing characters, giving them an identity and making them all really likeable. That translated over with WoW, with most of the world being built upon our understanding of WoW and not a clear new idea.
That really changed for Cata. Sure, twillight hammer, old gods and Deathwing was something from Warcraft, but it did not have the spotlight in Warcraft 3, which made it so that Blizzard have to create their identity from square 1. It then showed how the old formular of TBC and Wrath did not really work for these new, "original" expansions. Concepts were harder to implement and explain to players, which is why when MoP came out, the idea of the Sha was wierd. I still to this day think it is the weakest of all the enemies of WoW, because it did not really feel as a part of WoWs foundation or core. Instead of telling the story of Pandaria and the Sha before we got to experience them, we did at the same time. When we finally got the concept into our head, the expansion was over and it was time to think about time travel and WoD instead.
To point to something current, just look at GoT. I think everybody would say, that the story of GoT got worse and worse as they got further away from the books, as the producers had to spend more and more time on the foundation, instead of building upon something that was already there.
In the end, despite my clear problem-outline, i think the question is still quite complicated. Like a factor could also be, as the game has involved more story, it has become harder to make all of it good, compared to the past, where little story was told outside of a few quests in each zone. Several zones in the past had long quest lines, that told the story of the zone, but today, every zone has a HUGE quest line that leads you through the entire zone in one go. Many things can go wrong.