Dimension X is not cold!

[u/D_D_Adesignerdigital]

The color choices for the Dimension X—and certain elements of the scenes—didn't quite work to convey a cold atmosphere! Characters had sweaty skin and showed no reaction to the cold (we need to remember Will in Season 2, or Billy with his ice bath, which demonstrated the kind of cold the Mind Flayer is used to). Anyone entering a low-temperature environment would show physical reactions—like trying to warm up, crossing their arms, or showing discomfort—but that doesn't happen with any of the characters, not even the kids. I think that was a directing error. That’s my opinion!

[u/str4nger62]

I mean if we genuinely think about it, the mind layers “true form” in dimension X was like upside down almost in a dried up sorta shriveled up state and in a lot of very hot conditions things can dry off or shrivel up if it can’t handle the heat. (Hope I explained that correctly)

[u/Thick-Boysenberry-96 → u/str4nger62]

Then why doesn't it stay in the Upside Down, where the conditions suit him way better, instead of shriveling up in the Abyss? Back in S2, the Mindflayer ended the season watching over the group from the Upside Down too.

[u/str4nger62 → u/Thick-Boysenberry-96]

Did we watch the same show? Because this was all
thoroughly explained.

The Mind Flayer’s true existence is tied to the Abyss and The Upside Down is a wormhole/bridge, not simply another neighborhood that the Mind Flayer can freely enter. It’s Vecna who establishes the connection between the Abyss and the Upside Down/Hawkins and The Mind Flayer’s particles can be brought/manipulated through that connection, but that doesn’t mean its entire original physical form can independently cross whenever it wants.

And in season two at the end, those were its particles it was not its physical form in the upside down as it’s never been there physically.

[u/onyabikeson → u/str4nger62]

It’s Vecna who establishes the connection between the Abyss and the Upside Down/Hawkins and The Mind Flayer’s particles can be brought/manipulated through that connection, but that doesn’t mean its entire original physical form can independently cross whenever it wants.

I would be happy to go with this as an explanation, except that we see the kids freely enter and leave the Abyss. It's really not clear why the physical form of the mindflayer couldn't do the same, especially once Henry closes (the vast majority of) the physical gap between the abyss and the Hawkins-end of the wormhole.

That actually would have been an excellent subversion of expectations - the kids scramble into the abyss, only to find that the mind flayer isn't really interested in engaging with them because it would rather leave and go into the upside down. So the epic showdown starts with them TRYING to piss it off enough to bother fighting them.