Bolded are the overall ratings, italics is the original rating before S5 'tied everything together'.
Stranger Things S1: (8.5/10) 8/10 Overall a strong start, plucky kids, a desperate Mom looking for her lost boy and slovenly but determined and heroic local cop, a secret government lab and a kid with magical powers, this season hits a lot of strong notes and keeps the plot strands all running smoothly.
Stranger Things S2 (7/10) 6/10 A somewhat disappointing retread of the previous season, but at least at this point the characters seem to be progressing TO somewhere. On the downside the writers show that they care a lot about what Redditors complain about, which will become more apparent as the season continues, warping and changing as it does so.
Stranger Things S3 (5/10) 4.5/10 The clown season. With comically inept Rooskies somehow managing to build a secret science lab under a mall coupled with the disappointing Mind-Flayer monster. A lot of our heroes seem flat or just plain out of character and lazy writing underpins much of this seasons events/problems.
Stranger Things S4 (8/10) 5/10 A strong recovery from the clownish previous season with a return to a more serious tone and to its horror roots and with a strong 'new' villain and some new dynamic characters. The plot threads seemed much more disparate than before but, again, we're building towards something. The show seems to be going somewhere (it isn't).
There was a feeling of stakes with Max's impending fate and a compelling new mystery. Still, the satanic panic plot doesn't really go anywhere and is forgotten almost completely after this season. And a lot of the good this season is completely undone
Stranger Things S5 Overall 4/10 There's fluffing the landing completely and then there's retroactively damaging all of what came before by simultaneously over-explaining some things and not explaining others. The worldbuilding here is catastrophic, the plotting is sluggish, the effects are mostly overdone and awful. Too many characters, too little characterization, some characters stay dynamic, some devolve into shallow (and tired) tropes. Morally bankrupt decisions by our heroes and stupid decisions by the all-powerful Vecna makes it hard to care even about the complete lack of stakes. The 'epic climax' is almost embarrassingly easy and the epilogue self-indulgent and disappointing.
When many fans of the show say 'nah,. this wasn't the real ending, was it?' you know the writers done effed up completely. The behind-the-scenes doc was just the crappy icing on this overdone clown cake.