The original series, or the overall franchise, contains:
Anvilicious: The show, especially in Seasons 1 and 2, was very topical - addressing issues such as racism, alcoholism, drug abuse, etc. in a manner like Degrassi Junior High or after-school specials. In some episodes, these issues were handled very well. In others... to call them heavy-handed would be a massive understatement.
Ass Pull: The revelation of Jack McKay's survival in Season 10, despite the fact that he was blown up seven seasons earlier.
Awesome Music: The theme song! Or, at least, the two versions used from Season 2 onward.
Kelly, for her former reputation as the school slut, being an Alpha Bitch (though with Hidden Depths), and also the biggest reason, stealing Dylan while Brenda was away in Paris with Donna, which caused a major rift in the base. However, there are fans who believed Kelly improved over the seasons, became more of the Lovable Alpha Bitch, and it also helped a bit that she has a Trauma Conga Line (e.g. raped, nearly burned to death, suffering a miscarriage, getting addicted to coke, involved in a cult, shot), which made fans sympathetic towards Kelly. Heck, she has fans with Grant Show (Jake from Melrose Place), Jason Priestley (Brandon), and Sara Foster (Jen).
Dylan. Even though he is also the Breakout Character of the show, he did tend to act like a major Jerkass to both Brenda and Kelly when he was in relationships with them. It didn't help that he was often the loner who felt that he had to deal with everything alone, especially prominent in Season 5 when he lost his fortune, often rejecting the help of his friends even when they offered. However, his fans also stated that his Freudian Excuse of a con-artist dad and a New-Age hippy mom justifies his behavior, and also believe that he is genuinely kindhearted, well-read, and generous when he strips away all of his stoic armor.
Donna. Due to the nepotism of the character (she was played by Aaron Spelling's daughter and pushed heavily after the first season due to this) and longstanding rumors that Spelling, as one of Fox's most important show runners of the 90s, demanding that no show on the network could make fun of his daughter, after Julie Brown mocked her in a short-lived variety comedy show on the network. In her defense, Tori Spelling does give it her all acting-wise and the show quickly retooled her character to be a plucky comic relief figure.
Ray Pruitt. On one hand, he is a domestic abuser. On the other hand, the decision to make him a domestic abuser was done, simply to spite the actor because of his decision to leave acting/focus on his music career (which died as a direct result of said onscreen character derailment).
Brenda Walsh. The hate she received in the '90s was notorious, to the point of inspiring an "I Hate Brenda" newsletter and an entire album of songs bashing her. The most commonly cited complaint was that, as the show went on, her character went from a sweet Girl Next Door to a Designated Hero. Dislike of her actress Shannen Doherty also spilled over, especially as stories of her off-camera behavior (culminating in her departing the show amid reports of Hostility on the Set) began to leak out. And yet after she left the show, many people felt it wasn't the same without her. Retrospectively, there are many fans who actually prefer "the Brenda years".
Bizarro Episode: "The Time Has Come Today" in Season 4. In the episode, Brenda finds a journal in the floorboards of her room; the book was written by a girl who lived in the house during the '60s who was having friendship issues around the same time as the Vietnam War. Brenda connects with the girl because she is also at a rough spot with her friends at the time, and then goes on to imagine the girl Wendy and her friends as her and her friends. It's an odd episode because none of the characters look right as a '60s character (e.g. Steve in a Nehru jacket or Dylan as your typical hippie), and is basically an excuse for the writers to belt out their feelings about the Vietnam War, which none of the show's audience would really connect with. Also counts as Contrived Coincidence and Plot Parallel.
The post-graduation seasons, an improvement in quality or a sign of the show's decrease in quality.
Captain Obvious Reveal: Halfway through Season 4, a stranger shows up on Dylan's doorstep, claiming to be his late father's ex-girlfriend, and the mother of Dylan's half-sister, who he never knew existed. Dylan's agonises over whether this woman is legit, or if she is performing some sort of scam to get his money, and after half a season with her, and Dylan finally deciding that she is legit, we find out that she was scamming him all along and everything she said to him was just a ruse to get access to his money.
Diagnosed by the Audience: Emily Valentine's clinginess mixed in with her fixation with Brandon and self-destructive behavior (at one point, threatening to set herself on fire in Brandon's family's driveway) points to possible borderline personality disorder, though an exact diagnosis is never made explicit. She gets better, though.
Ensemble Dark Horse: D'Shawn Hardell from Season 4-5, while not actually being part of the main ensemble, was quite popular with fans, and regarding Donna, lead to the example below:
Even though Dylan/Kelly ended up being the endgame couple in the end, you'll find that many (if not the majority of) fans ship Brenda/Dylan and Brandon/Kelly instead.
Brandon/Andrea also has a popular following (especially with fans who don't like Kelly), they were ship teased several times in the early seasons, but it ultimately never went anywhere.
There are a good amount of fans who wanted to see Donna and D'Shawn (who were too ship teased) become an item.
First Installment Wins: Fans generally consider "the Brenda years" (seasons 1-4) to be the best.
Germans Love David Hasselhoff: The original series was very popular world wide and aired on practically every where where a European language is the primary one.
Denmark still airs this constantly for the past 30 years.
Not only are so many fans in Germany but the four of the cast won the German Bravo Otto Awards with Shannen Doherty winning twice. In addition Doherty also got a biography written in German published in the country while the show was at its peak.
In Japan in The '90s, judging with the considerable amounts of references in many anime series including Cowboy Bebop (which credits Jason Priestley and Shannen Doherty as the producers of a Show Within a Show) and even video games. Even Tomoya Asano, the producer of Bravely Default admitted he took some inspiration from this series for the game.
Growing the Beard: Because the ratings for Season 1 were poor, the producers sought for more change. They changed the theme, dropped Scott (permanently), elevated David and Donna's statuses in the cast, and the filming style was more soap-operatic than plain drama, reflecting the show's shift to season-long storylines with occasional very special episodes peppered in. It also helped that the producers aired Season 2 during the summer, when most primetime shows were off the air. Needless to stay, the ratings increased and Beverly Hills, 90210 gradually became a cultural phenomenon.
In the show's first season, Brandon gets in a drunk driving accident. Brandon's actor Jason Priestley crashed his car into a telephone pole while driving under the influence in 1999.
An in-universe example: in the Season 1 episode "Slumber Party" the girls discover diet pills in the one-shot Alpha Bitch Amanda Peyser's purse, which leads her to admit that she was fat and was taking the pills to keep her weight down. In the Season 3 episode "Perfectly Perfect", Kelly begins to have a major diet pill problem which lands her in the hospital on her birthday.
Another in-universe example. In one episode, David tells a guy who gave him a Jimi Hendrix tape that he particularly liked the song Manic Depression. David later finds out his estranged mother has manic depression and he is later himself diagnosed with it.
Valerie's backstory is that her father raped her as a child and her mother, played by Michelle Phillips, refused to believe her (though they did eventually reconcile). Several years later, her real life stepdaughter, MacKenzie Phillips, accused her father, Michelle's ex-husband and founder/lead singer of The Mamas & the Papas John Phillips, of the same thing, only she not only refuses to believe her (in spite of corroborating evidence), but to date they have yet to reconcile.
Hilarious in Hindsight: "I won't be gone forever, Dylan. Give me something to come back to." - Brenda Walsh, who never returned to the series.
Jerkass Woobie: Valerie has been raped by her father for years ( which led to her killing him in self-defense), had turbulent friendships and romances, her mother once disowned her because she believed that she was willingly sleeping with her dad and even tried to blame her for everything that happened to him.
Jerks Are Worse Than Villains: In later seasons, Kelly gets more hate than Valerie. Even if Kelly is supposed to be a Lovable Alpha Bitch who is loved by everyone, fans often see her as a self-righteous hypocrite who has no problem stealing her best friend's boyfriend. Valerie, on the other hand, is a slutty and manipulative Femme Fatale who likes to make everyone miserable... and fans basically worship her because she's a cool and entertaining "queen".
Not as extreme as the show's numerous parodies might have you believe, but it certainly has more than a few moments of this. The Sentimental Music Cue often used to underscore dramatic scenes certainly doesn't help matters.
The one-shot Alpha Bitch Amanda's freakout in the season 1 episode "Slumber Party" when the girls discover diet pills in her purse. Mostly because it comes about after most of the episode dealt with the other girls’ insecurities and traumas, and Amanda’s problem isn’t so as much as hinted at before the final five minutes where the pills just accidentally fall out of her purse when Brenda is trying to give it to her, making it appear shoehorned in for topicality purposes.
The scene where Scott accidentally shoots himself and dies in front of his best friend is supposed to be a sad moment that teaches a lesson about how careless use of firearms can have tragic consequences. But Scott being Too Dumb to Live makes the scene so ridiculous that it comes off as funny instead, to Black Comedy levels.
Ray pushing Donna down a flight of stairs. It's supposed to be a terrifying and horrible instance of Domestic Abuse, but the ridiculousness of the scene itself, the dialogue and her Obvious Stunt Double take away much of the drama of the situation.
Never Live It Down: The Ray Pruitt domestic violence storyline. Although neither the storyline nor the character lasted long on the show, it still is one of the first things fans remember about any relationship Donna had with someone other than David and the only thing to remember about Ray. It also arguably had a hand in killing Jamie Walters' acting career since he kept being associated with that role over anything else.
Nightmare Fuel: The conclusion of "Up In Flames". Aside from being one of everyone's greatest fears, the scene of Kelly and Allison trapped behind the encroaching flames, screaming for the firemen to hurry up and get to them, is truly terrifying.
Not Even Bothering with the Accent: One season 5 episode was meant to feature an angry death metal band from Australia who cause trouble at the Peach Pit After Dark. If your definition of Australian is Cockney British.
Older Than They Think: The iconic theme song associated with season 2 onwards actually made its debut in the pilot, set to a very lengthy montage of West Beverly High and the establishing shots of each character.
Matthew Perry appeared as a one-shot character in the episode "April Is The Cruelest Month". He would go on to play his famous role as Chandler Bing three years later.
Vivica A. Fox, before her breakthrough role in Independence Day appeared in "Ashes to Ashes". Also Eugene Byrd, who portrays squintern Clark Edison on Bones, appeared as Fox's on-screen brother in the same episode.
Paul Johansson, before One Tree Hill, itself a show about teens who looked in their 20's and...had problems.
Seasonal Rot: The exact point in which the show declined in quality varies for everyone. Most fans generally agree that it started to go downhill after Brenda's departure at the end of season 4. Dylan and Brandon's departures during seasons 6 and 9, respectively, could also be a contributing factor to this.
Season 3 has some shades of this, due to the Dylan/Kelly/Brenda love triangle, which caused a major rift in the fanbase. However, this has since been Vindicated by History.
Strangled by the Red String: Emily and Brandon have a brief, unconsummated relationship that ends after she slips him Ecstasy, resulting in her stalking and harassing him for several weeks after. Several years later, he tells Brenda she was the love of his life, despite the fact that he explicitly told her that he didn't love her when rebuffing her attempts at reconciliation.
Because TPTB decided to write in Gabrielle Carteris' Real Life pregnancy, but didn't want her to get pregnant by the first guy she slept with, Andrea abruptly dumped Dan, was rushed into a relationship with Jesse, got pregnant not two minutes after that—one of the very reasons she was considering an abortion—and married him not two minutes after that. We're supposed to believe that they're madly in love and ready to spend the rest of their lives together when they've only been dating a few weeks and would have likely never discussed marriage had she not been pregnant.
More like "Strawman Is Absolutely Right", with a little Designated Villain and Unintentionally Unsympathetic thrown in in the infamous "Donna Martin Graduates" storyline. The school administration explicitly warns the students that having or consuming alcohol at the prom is strictly forbidden and that anyone breaking this rule will be suspended, barred from graduation activities, and have to attend summer school. So Donna gets drunk, gets the punishment. . . and we're supposed to feel sorry for her and see the administration as the bad guys? Um, no.
Donna's mother Felice was also frequently portrayed as an overbearing bitch, and she was portrayed this way again during this storyline. But she has every right to be angry at the parents who served her underage daughter alcohol and at Donna herself, who drank the champagne.
They Changed It, Now It Sucks!: Some fans believed this after Brenda left at the end of Season 4. Furthermore, Seasons 5, 6, and 9 axed all the rest of the original characters - except for Kelly, Steve, David, and Donna. Also the reboot series.
Unintentionally Unsympathetic: Amanda from "Slumber Party". It's explained by the end of the episode that she's on diet pills and they have nasty side effects, such as mood swings, and it's implied that she's had a weight problem in the past (even at one point defending taking the pills as "Guys don't like fat girls!"), but it ends up being hollow due to her spending the night being as unpleasant as possible. Among her troublemaking and overall rudeness include teasing Andrea for having unrequited feelings for Brandon, implying Donna is a wuss who can't handle life and goading Kelly, her so-called friend, to talk about the time she was date raped (which reduced her to tears).
Unpopular Popular Character: Being the Token Evil Teammate, Valerie is usually disliked by most of the main cast. However, fans find her cool and interesting, and love how she is played by Tiffani Thiessen. That's saying a lot, since Valerie replaced the even more popular Brenda. It helps that the character who hates Valerie the most in-universe is Kelly, who is... very divisive among viewers.
Values Resonance: The topic of the season 2 episode “Ashes to Ashes” deals with the topic of anti-Black racism, specifically "walking while Black", which comes off as eerily resonant in the 2010s and 2020s, as systematic racism and its effects on communities of color are being brought to the forefront of national conversation.
The Woobie: Almost all the characters at some point or another:
Brenda when she broke up with Dylan (because he cheated on her with Kelly, her best friend), and was robbed at gunpoint.
Kelly, for her issues with her mom and dad, losing her virginity to a Jerk Jock who left her and spread rumors about her, her rape, her near-death experience in a fire, her miscarriage, her involvement in a cult, and so on and so forth.
Andrea remained a virgin longer than most of the other characters, but immediately when becoming sexually active she became pregnant, giving birth to a premature daughter who nearly died, having little support from her parents when she decided to marry Jesse.
Dylan had a con artist dad and a runaway New Age hippy mom, was The Alcoholic and a drug addict prone to relapses, his dad was killed in a bombing in front of him (though this was later retconned), he lost his fortune to his father's ex-mistress who then took her daughter (Dylan's half-sister) to Brazil, he lived homeless for a while, and lost his wife in a drive-by shooting meant for him, ordered by his father-in-law.
David was dealing with a womanizing dad, a not-so-there mom (although she did get better), watched his best friend accidentally shoot himself on his birthday and recurrent drug problems.
Donna had drug problems, parental issues, domestic violence by her boyfriend, nearly got raped, was stalked and held up at gunpoint, and witnessed her father's death from a heart attack.
Gina in her final appearance at least, where everyone blames her for Dr. Martin's death from a heart attack, though she was simply just there when it happened. Even Mrs. Martin insinuated that her very existence placed a strain on his heart, which caused his death. Luckily, everyone realized their mistake after Gina did the eulogy for Donna when she couldn't do it.
The sequel series contains:
Ass Pull: Silver's cancer diagnosis in the series finale, the episode after she had a miscarriage.
Breakout Character: Liam didn't appear until the end of the first season, but by season 2 he was basically the male lead.
A canon version with Riley's death paving the way for Annie and Liam to get back together.
This was actually not the case with the fans. While they didn't like him for dating Annie, they really didn't have anything against him.
Ensemble Dark Horse: Mark Driscoll. At least, people who comment on him consider him one of the most interesting minor characters, and have shown that they want him to appear more - maybe that's why he still is.
Growing the Beard: Season 1 was considered a by-the-numbers teen drama, resulting in mixed reviews (though its ratings were actually the highest when compared to later seasons). Season 2 however saw less original main characters, the departure of Ethan, increased writing quality, and more added fun, which continued into season 3.
Heartwarming Moments: More of a meta example but the "90210 4Ever" retrospective that aired after the last episode showed how close the cast had become. Unlike the Gossip Girl retrospective, the cast do their interviews in twos or threes, allowing them to laugh and banter with each other; Shenae Grimes and AnnaLynneMcCord spend most of their shared interview holding hands. They also pass around a phone specifically made for "90210 selfies". Unlike many other specials of this kind, it doesn't feel fake and you can really see that the cast had fun making this show.
Hilarious in Hindsight: In Season One Naomi says if she were a lesbian, she would date Adrianna. In season 2, a lesbian does date Adrianna.
Also in Season One, George, played by Kellan Lutz, is on the Lacrosse team. Lutz would later star in a movie about Lacrosse called "A Warrior's Heart".
Harry gave his son up for adoption and later adopted Dixon, who is black. In Season 5, it's revealed Harry's grandchildren from said son are half-black.
In the fourth season, Liam's friend Jim, played by Stephen Amell returns from being lost at sea.
In the fifth season episode "Con", the cast goes to a convention that makes refrences to both Star Wars and Marvel. Matt Lanter, who plays Liam, also provides the voice for Anakin Skywalker in Star Wars: The Clone Wars, and years later, Jessica Stroup, who plays Silver, would star in Marvel's Iron Fist (2017).
Moral Event Horizon: Many fans feel that Adrianna is irredeemable at this point. After switching out Silver's medication for Bipolar Disorder with placebos, it's going to be very hard to make Adrianna likable ever again.
Nightmare Fuel: Naomi's rape at the end of season 2. Especially Harsher in Hindsight, when in 2014, Naomi's actress AnnaLynneMcCord revealed that she was sexually assaulted by a friend when she was a teenager.
Questionable Casting: Ryan O'Neal as Teddy's douchebag father? Considering how good of a father he is in Real Life, that is just wrong.
Rescued from the Scrappy Heap: Annie dating Liam is what got the fans to finally accept her before she eventually became the most popular character on the show.
Teddy is much better received when he received the Coming-Out Story.
Retroactive Recognition: Stephen Amell played a presumed dead guy who returns after being lost at sea. One year later, he plays the same type of character as his breakthrough role, Oliver Queen on Arrow.
Seasonal Rot: Season 4 and 5 are considered lower in quality because of its elevated melodrama, Flanderization of certain characters, backstabs galore, and too much focus on several relationships, especially Liam/Annie. It doesn't help that the series finale had an Ass Pull with Silver's cancer diagnosis.
They Wasted a Perfectly Good Plot: They should have shown Adrianna's mom while she was dating Gia, based on Maeve Quinlan's other role. Even if she was completely supportive of it it could have been a great contrast
The Woobie: Annie at the end of Season 1, leading people who previously hated the character to instantly love her. Or at least gain acceptance in the fandom. Though now it's getting rather old.
Silver suffers horrible tragedy after horrible tragedy it borders on ridiculous. This website lists all of Silver's indignities in quick succession.
and an entire album