I'll repost what some anon said in the other thread:
after a solid amount of foreshadowing, the spooky conspiracy cult The Blind Eye Society was finally stumbled upon, and immediately made no longer a threat in a single episode.
a picture in the Northwest Manor implies that the family has ties to Bill Cypher
Pacifica's dad both fails to recognize Bill as that thing he has paintings of in the mansion, and while he does pledge himself to try and save his own hide no mention is made by him nor Bill about the Northwest family.
Despite selling it as a brother-sister duo, in execution its clear Hirsch only saw Dipper as the main character. Mabel gets only 1/10th the character development he does which makes Mabel seem like a way more despicable person than was intended.
A villain who is just sitting on their throne and only ever reacting to the hero's attempts to thwart him seems borderline harmless compared to a villain that actively pursues the heroes. This is why Bill felt so underwhelming in weirdmageddon compared to before.
approaching the end of the series, a minor recurring antagonist, Pacifica, has two nonconsecutive episodes that indicate she is going to become a proper ally, like Soos, Wendy, Candy, and The Thing.
That gets completely dropped and the way she talks in the grande finale its like those episodes never happened
a Big Deal like the author finally coming back typically heralds that the story has entered Endgame. That's the time for shit to get real. One breather episode is okay, but almost nothing but breather episodes until the finale is not.