Just finished watching this with my wife. I enjoyed it a lot, but I was thinking how does shutting down the portal gun so the door reseals kill the Mind Flayer? It was able to survive on our side due to the piece of Mind Flayer mind/soul/spirit (whatever you'd call it) which they got out of Will last season, and that part didn't die when El sealed the door at the end of S2, so why does it all of a sudden die this time?
I also found it a bit annoying that they didn't stop to call the military or secret service about the big Russian secret base at any point, and the fact the military turned up so soon after they were done kind of rendered everything they'd just accomplished pointless. Like, they could have just gone and hid somewhere for a few hours and Paul Reisar and backup would have taken care of everything about 15 minutes after they did anyway.