While the panic and desperation Joyce feels in Season 1 — not only in trying to find her missing son, but also for how no one believes her when she tries to tell them of the supernatural elements of his disappearance — is very justified, her constant hysteria pushed her into this territory for some viewers. Fortunately she hardens up with every passing season.
A certain moment where Dustin describes Eleven knocking Lucas unconscious, to the point where everyone was briefly afraid he had been killed, as awesome. To his face.
Early on, it may seem a little hard to feel sorry for Nancy's overwhelming concern for Barb after her disappearance, considering that she did have her best friend personally drive her to a party Barb didn't want any part of, and then went to blow Barb off to have sex with Steve afterwards. This caused Barb to stay behind out of concern for Nancy's well-being due to her having been drinking during the party, leading to Barb being abducted and killed by the monster shortly afterwards. In fairness, Nancy had no way of knowing that a monster was lurking around the town kidnapping people, and the timing of the attack was unfortunate, but Nancy's actions and initial dismissal of Barb did indirectly lead to her being taken and killed by the monster. She becomes much more sympathetic come Season 2, where she acknowledges all of this, feels guilty about it and redeems herself for it by getting closure for Barb's parents.
On the flip side to the above, there are viewers who didn't feel much sympathy for Barb herself in this situation. While her concerns about Nancy going to Steve's party, drinking beer, and possibly getting asked for sex by Steve were certainly valid, some viewers feel they weren't actually out of Barb's concern for Nancy's wellbeing but out of fear that Nancy could get in with the "cool" crowd and thus become too cool to hang out with her, and Barb didn't want Nancy to change herself too much so that she could still be her friend. Barb's line of "Nancy, this isn't you" in response to Nancy making her own decision based on what she desired and thought was best for herself at the time (to have sex with Steve) is often cited, as is Barb sitting around moping by the pool even when she was told by Nancy that she could go home at least twice (which ends up leading to her death) and her attitude towards Nancy wanting to lose her virginity coming off as Slut-Shaming.
Mike has been guilty of this in each season post-Season 1.
In season 2 regarding his behavior towards Max with his only reasoning being that he thought she was trying to replace Eleven, even with Max knowing almost nothing about her, as well as being dismissive and rude about her lack of knowledge about Dungeons and Dragons. His treatment towards Dustin and Lucas when they like Max is also unfair and awful, despite them supposedly being two of his oldest friends. And then, after he sees Eleven again, he tears into Hopper for hiding her away, despite him doing it primarily to keep her safe and away from the reach of Hawkins Lab, and refuses to listen to Hopper's reasoning, just yelling at him about how unfair it was that he didn't get to see Eleven. And when Steve is trying to keep them inside the house and safe from the demo-dogs, Mike just insults and ignores him.
In Season 3, he and El regularly ditch their friends to go make out, even when Dustin has just returned from camp. When Will calls him on it, he just responds that it's a typical part of growing up; ignoring that Will's issue wasn't that they had girlfriends, but because Mike was constantly ditching him and when he was hanging with them, he was clearly only doing so out of obligation or because El wasn't available.
In Season 4, he acts cold to Will and has barely written or called to him, in marked contrast to how often he speaks to El - and when he's first called on it, he says it's Will's fault for not making more of an effort. Later, he blames the bullies' attack on Eleven at the rink on Will by accusing him of acting depressed and "ruining the whole day."
For some viewers, the attempt at Redemption Equals Death for Billy in the Season 3 finale doesn't land, with critics arguing that Billy didn't redeem himself at all, and deep down was the same sociopath that we'd seen in Season 2. Part of the problem is that the things some fans were upset with Billy over (nearly running over three kids out of sheer pettiness, his emotional abuse of Max, beating Steve into unconsciousness, etc.) were glossed over, with Billy never really making an attempt to atone for his vile behavior prior to his death.
Karen Wheeler in Season 3 falls into this since she considers cheating on her husband with Billy (who is roughly the same age as her daughter Nancy). Granted her husband is lazy and unappreciative and she does ultimately change her mind, there's still the fact that she, a grown woman, thought about cheating on her husband with a high school student.
Hopper's behavior in the first three seasons of the show could come off this way to some fans who aren't fond of his short temper and tendencies to get violent with people in order to further his investigations, to say nothing of the show glossing over the fact that he sold out Eleven and the other kids' location to the lab agents in the first season finale, while his harsh treatment of Eleven in season 2 has been viewed by some as bordering on emotional abuse.Season 3 is generally considered to be when Hopper is at his worst, as his abrasiveness is blown into full-on aggression at the drop of a hat. He acts very possessive over Eleven, threatens Mike into staying away from her and is condescending and belittling towards Joyce despite trying to pursue a romantic relationship with her. His violent fury when she stands him up on their date (under the incorrect assumption that she was with another man) is somewhat Played for Laughs but was too much for many viewers, who felt that it was veering dangerously close to Domestic Abuse. This isn't helped by the fact that the audience is expected to sympathize with his reasons for acting this way despite these reasons not being explained in any depth until the end of the season finale, making it hard to understand why he's being such an asshole beyond "Mike is annoying and he's pissed at Joyce".
That said, it was in season 3 that Hopper fell into this trope for a lot of fans, who felt that the show's reliance on 1980s "action hero" tropes caused him to come off like a walking bundle of relationship red flags. A couple standout examples in particular:
Hopper is bothered by the relationship that Mike has formed with Eleven. Joyce suggests that Hopper have a heart-to-heart with the kids and talk out his issues with them. He deviates from the plan by luring Mike into his car to talk to him. What starts as a Twerp Sweating gets worse as he proceeds to intimidate and threaten Mike. Mike is clearly very much afraid of Hopper because he's a teenage boy trapped in a car with a grown and armed man who is three times his size. When Mike is too scared the next day to meet Eleven, Hopper sings happily that no one dares to mess with him, after seeing Eleven in tears because he's not coming to see her. And while Mike may have been arrogant to Hopper before, a lot of that could arguably be viewed as Hopper's fault, given that he wasn't being very respectful to Mike either. To some viewers, Joyce standing him up when he tries to ask her out on a date feels like a karmic punishment for him.
In episode 4, Hopper barges into Mayor Kline's office and interrogates him into giving up information, breaking his nose, and threatening to cut off his finger. Sure, Kline is a Corrupt Politician and an arrogant asshole who's in bed with the Russians, but what Hopper does with him borders on torture. Although Hopper is supposed to be someone who proves that Good Is Not Nice and has shown a capability for violence in earlier seasons, he generally used it as a last resort (such as knocking out the guard at the morgue to get to Will's "fake" body in season 1 after his attempt to bluff said guard failed). Him being quick to resort to violence in this scene doesn't reflect well on him, especially considering that Kline is much weaker than he is and in no position to fight back.
To be fair, he'd just seen the guy who beat him unconscious the night before walk out of Kline's office.
His behavior towards Joyce in general has been viewed by some fans and reviewers as coming off less like Belligerent Sexual Tension a la Sam and Diane, and more like the actions of an abusive and controlling boyfriend, such as refusing to listen to Joyce's reasons for standing him up on a date when she went to see Mr. Clarke, trying to push her into dating him while she's still clearly mourning Bob's death in the previous season, and then proceeding to belittle her, gaslight her, and outright call her crazy (a sharp contrast to the Hopper of seasons 1 and 2 who was firmly on Joyce's side and was a constant source of moral support for her and her family).
caused him to come off like a walking bundle of relationship red flags. A couple standout examples in particular: