I feel like a lot of their issues with writing can be summed up as writers who fear they’re on the clock and have one last big story telling chance to wrap up everything they want to touch on. It’s like writers for a TV show that have been told it’s their last season who hastily try and close up all the major loose ends and sub-plots, often with less than satisfactory results if it’s been a long-running show. Same thing here, they were convinced they’d struck out and that it would be over soon[tm] so they went about trying to wrap things up for characters they cared about or at least tell some compelling stories with them before it was all turned off.
That and the issue with every expac basically being a world-ending threat that has to be stopped which after a while just gets stale and with Blizzard’s quality of writing just isn’t compelling or emotionally gripping in the way they want it to be after the 5th time. Especially with how compressed the timeline is, with all these things in lore taking place in such a tiny span of time without more mundane plots between them to space it out.
The problem is this mentality I think carried on or cropped back up at least through WoD. Legion seems like maybe they got more in the mindset that they had time left and they wanted to take a radical departure and start killing off the long-time characters or moving them off to the side. That could be more of fearing it’ll be over soon too though, it could also be more of a genuine attempt to reinvigorate the story by casting off some old baggage and setting sail in a new direction. Maybe some of both.
Sylvanas is the opposite problem, taking a character’s story, milking it for way too long, and the player-base knowing their writing cannot support this kind of buildup and groaning all along the way. It would have helped if there were more compelling, fleshed out, towards center stage side-plots along the way but much as they try and insert other characters and little stories here and there, at the end of the day the only threads left standing and still getting attention are certain main ones. And that’s a whole other problem with Blizzard dropping interesting threads and just ignoring them in favor of some narrow-sighted vision for a specific story they want to tell.
MoP was actually a break from this in many ways. Or at least it felt like it wasn’t the same hatchet-job stuff we got before and after, it was something new, something independent. It wasn’t on crutches all the way talking about some main character or world destroying villain (not until the final act), it was about saving these people and their one land and that was compelling. Up until the final act of course.