Anvilicious / Some Anvils Need to Be Dropped: Zig-zagged. The show, especially in seasons one and two, was very topical - addressing issues such as racism, alcoholism, drug abuse, etc. in a somewhat After School Special-like manner. 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. Prime Time Soap, indeed.
Awesome Music: The theme song! Or, at least, the two versions used from season two onward.
Dylan. Even though he is also the Breakout Character of the show, he did tend to act like a major Jerk Ass 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.
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 too 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 wouldn't really connect with. Also counts as Contrived Coincidence and Plot Parallel.
Draco in Leather Pants / Ron the Death Eater: Brenda's fans would have you believe that Kelly ruthlessly stole Dylan from her best friend while kicking puppies and laughing about it, while Brenda saved bunnies in Paris. Kelly's fans are slightly less extreme in their view of Brenda, but they do paint Kelly somewhat more sympathetically than they perhaps should.
Kelly: for her parentalissues, 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.
Valerie: Getting 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 Val's dad.
Narm: 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.
Most of the original teen characters were in their junior year in season 1. However, they were still in that grade for season 2. The producers decided to extend the lifespan of the then-fledgling show by retconning the characters' ages, except for David, who was already a year younger than them.
The death of Valerie's father was retconned several times. Initially presented as a sudden suicide, Valerie later admitted she killed him in self-defense when he tried to rape her, and then it was revealed that she straight up murdered him without provocation (aside from the aforementioned rape). However, according to Jim Walsh, it was sudden, so....
Jack McKay's death is a major retcon, since his car was shown blowing up with no apparent way of escaping. Season 10 retcons this with Jack having survived, and entered the Witness Protection Program in the aftermath of it all.
Seasonal Rot: Same as Broken Base above, some fans believed that after season 3, the show's quality started to decline, though others believed the show increased in quality after the characters graduated.
Tear Jerker: Brenda learning that Dylan cheated with Kelly (season 3's "Back in the High Life Again"). Even though Brenda knew Dylan had an affair with someone during the summer, she didn't know who with. When Kelly revealed that she was the one, it was the ultimate betrayal. Brenda losing her boyfriend to a person whom she thought she could trust, which could easily connect with viewers who had a similar experience, and also goes to show that betrayal is never at the hands of your enemies. It doesn't help that Dylan acts very blase about it, though Kelly tries to soften the blow, though Shannen Doherty's performance speaks volumes.
Kelly: Neither of us wanted to hurt you.
Brenda: You didn't care how I felt at all, Kelly!
(couple lines later)
Brenda: Look, I hate you both. Never talk to me again!
Dylan: Con artist dad, runaway New Age hippy mom, 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), lost his fortune to his father's ex-mistress who then took her daughter (Dylan's half-sister) to Brazil, lived homeless for a while, lost his wife in a drive-by shooting meant for him, ordered by the woman's father
David: Dealing with a womanizing dad, a not-so-there mom, recurrent drug problems.
Donna: Drug problems, parental issues, domestic violence by her boyfriend, nearly getting raped, stalked and held up at gunpoint, witnessing 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.
Complete Monster: Douglas Atherton is a wanted rapist from England. He comes to America, renames himself Miles Cannon, and becomes a teacher to find new victims. At the end of season 2 he rapes Naomi Clark, even telling her that because she lied about him sexually harassing her earlier, no one will believe her about her rape. After he is found out, he flees his apartment, only to confront Naomi later and threaten her with a knife.
Creator's Pet: Annie could be considered this, particularly in the earlier seasons. In season 1, Naomi told Annie she would be uncomfortable with her dating Ethan since he was Naomi's ex. So what happens? Annie dates Ethan behind Naomi's back. The writers then expected the audience to feel sorry for her when Naomi hooks up with one of Annie's exes at her Sweet Sixteen party. The majority of fans sided with Naomi instead. Annie's Pet status continued into season 2 when she took the ENTIRE season to confess to her hit and run, but ends up only being put on house arrest for a couple of months (again, we're expected to sympathize). During that time, some fans even criticized Shenae Grimes, when Grimes was just doing her job. By the later half of season 3, the Annie hatred started to die down due to her screen time being cut significantly.
Ironically, it was Naomi who became this in later seasons. She continued to get focus even though she usually made things worse for everyone.
Crowning Momentof Heartwarming: 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 AnnaLynne McCord 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.
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 Darkhorse: Mark. 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.
Fan-Preferred Couple: Annie and Liam have become this. It's weird because they experienced Foe Yay in Season 1, became friends in Season 2, started dating in Season 3 with a few breakups in between and broke up in Season 4 and fans want them back together. Against all odds, they overcome this and ascend to Official Couple.
Foe Yay: Annie and Liam in Season 1. It fades over time and long before they actually start dating.
Arguably Annie and Naomi in Season 1 and 2.
Naomi and Austin are a canon version.
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.
Season 4 and 5, however, are considered lower in quality because of its elevated melodrama, Flanderization of certain characters, backstabs galore, and too much focus on several relationships, including the Liam/Annie shippers. It doesn't help that the series finale had an Ass Pull with Silver's cancer diagnosis.
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 Kellan Lutz is on the Lacrosse team. Lutz would later star in a movie about Lacrosse called A Warriors 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.
Magnificent Bitch: Both Naomi and Annie have shown shades of this, Annie probably from hanging out with Naomi.
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 AnnaLynne McCord revealed that she was sexually assaulted by a friend when she was a teenager.
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.
Retcon: Silver's age. She was born at the start of the original show's third season (1992) yet graduated with the class of 2011. With summer birthdays, students usually graduate before their 18th birthdays, though Silver would've had hers before the beginning of season 3 (her senior year), instead of season 4 (college freshman year).
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