Just watched seasons 1 and 2 for the first time. Pretty good show, tbh. Season 2 was not quite as good as season 1, but still thoroughly enjoyable. There was just a lot less mystery around season 2. One of the great plot functions of season 1 was Eleven's constant flashbacks to her experimentation, which slowly revealed the extent of her powers, the motivation of Hawkins Lab, and what the the upside down was. Season 2 had a lot less of that.
I really like the evolution of Steve... in season 1 he was a trying to take advantage of a nice prude girl, but by the end of season 1 you realize that nancy was basically a and steve actually changed for the better. Steve mentoring Dustin was pretty cool, too.
My main criticism of season 2 was Eleven's side story. They tease Kali in the beginning of season 1, then don't touch up on her until midway through the season, where she basically has 1 episode and then eleven just leaves anyway. All that buildup for Eleven learning to focus her powers? btw, that scene was basically a remake of Xavier helping Magneto in X-Men First Class