Just started and finished the show a couple days ago, and absolutely loved it.
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First off, the general tone was amazing. It was like if you were to have Stephen King write a show based off of the X-Files and Twin Peaks and then were to give that screenplay to a young Spielberg to direct. I also really liked the explanation of the upsidedown as the sort of parallel, sort of the same universe. Made sense as to how Will was able to interact in real time with the regular universe.
Acting was superb on all counts, especially the children. They definitely sold the show for me.
In regards to season 2:
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I'm actually glad that they're keeping the story going. Right now it's definitely becoming more and more the trend to have dark themed anthology style shows. I'm glad to see that the Stranger Things team isn't just jumping on that bandwagon. Personally I'll be really happy to see these characters being developed and fleshed out more. I'm sure that as far as the universe of the show is concerned, there will always be something new that is up for interpretation, even if other things get answered.
One thing I thought was weird:
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For a show that takes almost all of it's inspiration from 80s cinema, the upsidedown sure looked and acted a lot like Silent Hill.
I saw that today. I didn't understand her reactions. It was almost like making a mockery of your cast and show? She's an odd one for me though. I wasn't totally in love with her from the show anyway.