They COMPLETELY botched the main villain!
Seriously? We go through the entire show with the plot revolving around the mind flayer and then all of a sudden the show takes a three year hiatus and this is the twist that they come up with?
Some low key wizard puppetmaster that they just introduced at the tail end of the show, has been manipulating a giant fucking sand spider this whole time?
How do you just sweep all of the build up of one of the coolest, most ominous bad guys under the rug by reducing it to nothing more than conjured fantasy by some psycho kid?
It felt so rushed and incoherent. No hinting of a big bad behind the mindflayer.
No subtle nods foreshadowing of a villain behind it throughout the entire series.
And then just out of nowhere, here's this new dude that we decided on making the main villain of season 4...oh and also he was secretly the bad guy this whole time since the beginning. Like they struggled to put a face to the upside down.
The upside down didn't need a face. It worked perfectly because the mindflayer HAD no motive. It was just this destructive force of nature that consumed. No different than a tornado.
This has got to be the worst retcon since all of season 8 of GOT.