SFBadger,
I completely forgot about the Russian takeover thing. Yeah, that was really bad. Like you said, believability should be suspended only at the genre level. If this were a Spy Kids movie or something, sure, let the Russians have a secret base under a mall on domestic soil that the NSA, FBI, and CIA all totally missed. In Stranger Things, I expect better. And a trio of kids foiling them at multiple points along the way wasn't any better.
I think the mystery of what all was going on is a big part of what captured my attention in s1. It can be hard for shows founded on mysteries to keep up the same level of interest when enough of the initial mystery has been revealed. Still, I thought s2 handled itself admirably. s3....like the second Highlander movie, I'm just gonna pretend that didn't happen