Dado o pouco que sabemos sobre o Upside Down, é mais rápido listar as semelhanças:
They're both dark, shadowy reflections of the "normal" world, and places of death and decay.
Além disso, se quisermos compará-los mais profundamente, as respostas tendem a ser "realmente não sabemos", então não há muita comparação a ser feita. Afinal,
we were only there for a few minutes of screen time total, and we saw all of the thematically important things: rot, fungus, weird stuff growing over everything, decay, death, etc. We also only saw one inhabitant: the thing the kids called the Demogorgon. There's a lot of suggestion at there being much, much more, but really the Upside Down we saw was pretty simple in terms of components.
Existe uma clara diferença:
Unlike the Shadowfell, which is quite significantly departed from the shape of the Material Plane, as best we can tell it seems the Upside Down extremely closely mirrors everything in our world, down to the placement of individual lawn chairs. It must look pretty strange in there when we're busy building something.
Poderemos fazer uma comparação melhor quando houver uma segunda temporada, mas até lá não há muito a ser feito.