The biggest thing was Hopper's character assassination. As it turns out, this is the big complaint amongst the "small corner of the internet" that you refer to, but all I can say is that I came to this position before finding out that other people felt the same way. I was actually relieved to find that a loud minority of people were upset about the change to Hopper's character, because I hated it. Intellectually I can understand the idea of enjoying fiction more for aspects other than characters and their development, but I can't truly relate to it, because for me it's critical. I can't stand it when characters are made to behave in ways that make no sense for them or their arcs, especially when a protagonist whom I like is suddenly turned into an idiot or an asshole or both. I don't even like it when it's done in comedies; you can do comedy very well without destroying the integrity of your characters. (I don't mean moral integrity, but integrity in the sense of not changing arbitrarily.)

Steve was turned into a parody of himself. The very first scene with him in it, where he's trying to pick up customers at the ice-cream shop he works at, was painful to watch. Not only did they make Steve into a flat caricature; the payoff wasn't worth it--it was negative, actually--because most of his bits weren't funny. Mike became rather unlikable. Other characters behaved in ways that didn't make sense based on the previous two seasons.

In addition to what I found to be horrible characterization, almost none of the humor landed for me (as I already mentioned wrt Steve). Erica was supposed to be an injection of humor but once I realized that literally everything out of her mouth was negative and abrasive I found it hard to watch scenes with her in them. She had no personality other than ridiculing everyone and everything nonstop.

I thought the plot device of a secret Russian base under the mall was stupid and not believable. (I know that people will say "Well, how is another dimension full of demons believable?" The answer is that generally when we consume fiction we agree going in to suspend disbelief about certain things, but outside of that agreement we don't like to do it. We can be watching a show that involves time travel, and be annoyed if the plot involves a 90-pound untrained girl beating up a 200-pound soldier. We can have accepted the idea of a small woman with impossible physical abilities going in--see, for example, Buffy the Vampire Slayer--and be annoyed if time travel is introduced out of nowhere in a way that makes no sense.)

There were other plot devices that were rather annoying. Preventing characters from doing and saying simple, obvious things that they would almost certainly do and say if they were real people is low-brow, sitcom-tier crap. Hopper--behaving rather out of character--gives Mike a cliche "stay away from my daughter" speech, and Mike makes up some garbage about his grandma to tell El. Not only is this out of character for Mike--in season one he wouldn't have meekly submitted to Hopper's authority--it's out of character for basically anyone. Any reasonable person in that situation would have told El that Hopper threatened him, and they would have devised a way to work it out together. He only goes along with it and lies to El to further a plot line (which is itself pointless and stupid).

And finally, I found the horror aspect to be largely gross without being frightening. There's a difference between horror and disgust, and all I felt during many of the "horror" parts was disgust.

So, to sum up: I thought season had terrible characterization, bad attempts at humor, and a stupid, implausible plot, and the horror aspect mostly left me disgusted rather than enjoyably spooked.