The original series, or the overall franchise, contains:
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.
Awesome Music: The theme song! Or, at least, the two versions used from season two onward.
The post-graduation seasons, an improvement in quality or a sign of the show's decrease in quality.
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.
Fan-Preferred Couple: 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.
Germans Love David Hasselhoff: The original series was very popular in Japan in the 90s, judging with the considerable amounts of references in many anime series and even videogames. Even Tomoya Asano, the producer of Bravely Default admited he took some inspiration from this series for the game.
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.
Retroactive Recognition: 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 3 years later.
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 raped Naomi Clark, even telling her that because she lied about him sexually harassing her earlier, no one would believe her about her rape. After he is found out, he flees his apartment, only to confront Naomi later on 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 Anna Lynne Mc Cord 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 overbecome 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.
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.
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.
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