I think a major thing is, and maybe someone has touched on it and I missed it, but the issue with the story telling, is that it is no longer through the lens of the Heroes like it was in the RTS games. (WC3, etc…)
In WC3, you played as the main characters. You were Arthas, Illdian, Jania, etc. You got to play and experience those character’s story arcs, which made them seem fulfilling. That’s what gave you the background in their stories.
Imagine a WC4 was released and you got to play as Garrosh, you commanded armies, won battles and played through his eyes. The story and his character development would have been so much more personal.
Imagine a WC5 was released and you got to play as Slyvanas, you threw yourself from the top of ICC and worked your way through the Maw. You met the Valkyries and experienced her story, all those behind the scene deals and you understood her character. (Remember playing Illidan and Tyrande in WC3?)
We do not get those same feelings now. We don’t get to see character development through the character’s eyes. We have to read comics, books, youtube videos, etc… to try and piece all this stuff together.
That’s usually where a lot of retcons come up, there are so many platforms that Blizzard themselves cannot keep track anymore.
WoW Vanilla was great, because it was an established world, from WC3, that you were just thrown into. You played the part of a peon, and it worked because the looming threats of WC3, the more “adult” (illidan, Kael, Vashj, etc) problems were beyond your scope.
You weren’t a “hero” and that was above your pay-grade.
In BfA, Drustvar was great because you went back to being a peon. You weren’t some overlord planet god killing titan. You were helping a town with a Witch problem.
Come the end of Vanilla, and into TBC, you started to become the focal point of the story because Blizzard was no longer going to produce RTS games. This is when the story began to downward spiral.
TBC had awful lore, and completely ruined a lot of major characters, if I remember correctly. It didn’t feel like it did Illdian, Kael and Vashj justice. But it had a strong tie in with WC3 that kept players around.
WotLK had a very similar aspect to it. It was Arthas. THEE Arthas. It wasn’t an exceptionally great story either, but people wanted to see the story through because it was such a huge aspect of WC3.
After WotLK, the player base dropped exponentially because of this. The story no longer had the backbone of the RTS games to keep it alive.
And as you moved into the spot light as, I’m the front and center hero this is now MY story, certain aspects of the game had to change.
This becomes a problem for two main reasons.
First, as mentioned before, the story is not told through the eyes of the “Heroes.” Slyvanas, Anduin, etc… and you lose interest in these heroes, you don’t get to experience the little things that make the character arch great.
Again, imagine you played as Anduin and you fought at the Broken Shore with Varian, you experienced the game play through his eyes and you couldn’t save your father. That feels so much more impactful.
Second, when this becomes YOUR STORY, you’d like to have choice in the matter. When things happen in the game, for an RPG, you want to be able to make “meaningful choices” when it comes to major plot devices and have those choices matter.
You don’t get that same feeling you do when you play a game such as SW:KotOR, or even Fable.
Now, the “story” becomes terrible because it compounds upon itself. You don’t experience these major character developments, and learn why they are doing it through their eyes.
Slyvanas and Zovaal aren’t interesting because we don’t know anything about what they are doing and why they are doing it. So none of it makes sense, to us.
That would all be fine and dandy, having secrecy, if our actions were our own. If we could make meaningful choices in the game that impacted the storyline to our liking.
You aren’t seeing their storyline and you aren’t given the ability to create your own. You are just following along as a mindless drone.