Finished the season. I think it has some really high highs, but is overall the weakest season.
The flayed was a wasted concept that had a lot of potential, but they just ditched it for a giant monster which was lame by comparison.
They really didn't lean on any of the mysteries this season. Most characters figure out what is going on very quickly, some questions are answered almost immediately. However there was some plot threads that were left just dangling. Why were the flayed eating chemicals? Were the rats infecting people? How come the rats never show up again after that one escapes the cage?
The first 5 eps were really great, but when it came to the climax in the last 3 eps the action wasn't that interesting. There were some great moments, like Dustin talking to Suzie, but it overall just didn't grip me. Alexei was an awesome character rip.
My favorite part of this season were the character interactions. These characters are all really fun, with a few exceptions, and their scenes are always a joy to watch. The plot and the conflict unfortunately just did not do it for me, and I was not as invested in what was happening as I was in S1 and 2.
Edit: And yeah as others have said Hopper's treatment in this season was really bizarre. Joyce's treatment was also weird by proxy.