1. Defeating Villains in a Satisfying Way
In order to create a feeling of danger and gravity, villains are usually more powerful than the heroes of their respective stories. In order for the heroes to defeat the villain, they need to use clever tricks, ingenious plans or magic. Caveat: the win condition needs to be introduced early on or it will always feel like a complete asspull.
We knew from book 1 / film 1 that in order to defeat Sauron, the heroes had to cast the One Ring into the fires of Mount Doom.
We knew from the start of WotLK that the Ashbringer could counter Frostmourne (the Battle of Light's Hope, the skirmish at the Cathedral of Darkness etc.)
We had no idea how to defeat Deathwing and introducing time travel and the Dragon Soul at the 11th hour felt like an asspull and people hated it.
Jon & co. had no idea how to defeat the Night King. Arya's sneak kill felt like an asspull and people hate it.
An even worse sin is if a way to defeat the villain is introduced but plays no role in the endgame.
Example 1: BtDP established that Deathwing's fatal weakness were his armor plates. But for some reason we couldn't just tear them off the conventional way and needed the Dragon Soul to defeat him.
Example 2: GoT established the mythos of Azor Ahai and the Lightbringer. Dragonfire (which is connected to valyrian steel) had no effect on the Night King but a valyrian steel dagger one shot him? Neither Azor Ahai nor the Lightbringer played any part in the Night King's defeat.
But the worst sin is if the writers retcon the plot to make the villain's defeat make sense.
Example 1: Cataclysm established that the Aspects' great purpose was to defeat Deathwing (through combining their powers in the Dragon Soul I guess). In retrospect, empowering the Aspects was pointless because all they needed that power for was really just spending it to stop one of them. Tyr could've just told the Aspects to fuck off and Cataclysm would have never happened.
Example 2: Melisandre's throwaway line about Arya shutting brown, green and blue eyes in S3 was retconned to refer to her destiny to kill the Night King (D&D admitted as much in the post-episode featurette).
And this is how you don't defeat villains.
2. Satisfying Story Structures
There's two ways to write a story. Either you plan out a story with a clear beginning, middle and ending or you just YOLO it and see where things go. The downside of stories with a clear structure is that they have predictable endings. YOLO stories on the other hand generate a lot of fan interest and speculation because it is not obvious how they will end. And that's the problem.
You can definitely write a competent YOLO story, but it will never be satisfying. Writing the general outline of a plot is not hard, executing that outline well is. When you have thousands of fans speculating, they will inevitably come up with more interesting and intricate plots than the writers ever could. That does not mean they could write the story better than the original author. Some stories have an intricate plot but end up being complete blunders because of poor execution (see the Last Jedi). But when you have fans speculating and coming up with intricate resolutions to the story, the real resolution is never going to live up to the hype.
In LotR we knew that the story would end with a final showdown at Mount Doom.
In WotLK we knew that the story would end with a final showdown at the Frozen Throne.
In GoT we had no idea where the story was going or how the White Walkers were going to be defeated until Arya popped out of nowhere and shanked the Night King. None of the intricate fan theories about Azor Ahai, the Lightbringer, Nissa Nissa or the White Walkers' true purpose came true.
In BfA we have no idea where the story is going.
LotR was hailed as a great story. WotLK is considered by many to be an at least competent story. Yesterday GoT disappointed a lot of people. BfA is such a trainwreck that we know it will be a disappointment even without knowing the ending first.
Takeaway: if you invite fan speculation, you build up hype to silly levels. If your story can't live up to the hype then don't invite fan speculation.
Part 3. Mythos and Prophecies SUCK
Finally, we have some writers that introduce convoluted in-universe mythos and prophecies to create that feeling of "I wonder where this story will go" and invite fan speculation. The problem is that prophecies and mythos have to be intentionally vague or they'll give the ending away. But like we've covered above, fans can and do come up with more interesting interpretations for prophecies than the original writers.
GoT, like mentioned above, had Azor Ahai / Lightbringer / Nissa Nissa / the Prince that was Promised. D&D decided to just can the whole lot of them and didn't even bother interpreting them.
WoW has the Circle of Stars / Five Torches / the mysterious "She" mentioned in Old God whispers. The Circle of Stars was speculated to be some mysterious universe-shaping force. It actually ended up just being N'Zoth's prison.
If Il'gynoth had said "To find him, open his prison" it would've been immediately obvious what he was referring to.
"To find him, drown yourself in the Circle of Stars" sounds mysterious. But it NEVER could have lived up to the hype because it was always intended to just refer to N'Zoth's prison. All fan speculation was rendered moot and that's disappointing for people who had spent the last 3 years speculating about Il'gynoth's whispers.
The only "prophecy" that worked well was Illidan being the Child of Light and Darkness but it only worked because it was immediately obvious but was cleverly subverted in the end.
Takeaway: prophecies have to be vague to be interesting, but if you make vague prophecies then the fans will come up with better interpretations for them than the writers.
TL;DR N'Zoth will die to a lame asspull because there's no clear win condition for the heroes. Sylvanas' real plan will be disappointing because it will be something really simple and obvious, especially when compared to fan speculation. The rest of Old God prophecies will likewise be disappointing. The ending of BfA will be a complete shitshow because Blizzard is just blundering their way through the story instead of having a clear plan.
You know what other story had no clear endgame and ended up sucking?
Warlord of Draenor.