Base Breaker - Taylor Townsend, post-Heel Face Turn. She's either loved for her personality post-Character Development and promotion to the main cast in Season 4 (and for replaced Marissa as Ryan's One True Love), or hated for those exact reasons.
Crowning Music of Awesome - One episode usually consists of about five indie-rock gems, but "Hide and Seek", "Hallelujah", "Dice", "Life's A Song", "Something Pretty", "Running Up That Hill", "A Bad Dream", "If You Leave", "Champagne Supernova" etc. "California" is a given.
Ensemble Darkhorse - Taylor Townsend went from a minor villain, to a recurring friend of the main characters, to a main character herself, to Ryan's girlfriend.
Oddly, Summer Roberts. She was originally just suppose to have a small role in the pilot, and ended up being the main female lead of the entire series.
Alex Kelly is definitely one of the most popular characters but Executive Meddling made go away.
Fan-Preferred Couple - While Official Couple(s) Ryan and Marissa and (especially) and Seth and Summer are very popular in the fandom, many fans continued to ship Marissa with her short lived bisexual love interest Alex, long after she had dropped off the face of the Earth. It is also never made clear whether or not Taylor and Ryan ended up together, but the fans are nearly unanimous that they did.
The Seth/Anna shippers would like a word with you.
This troper fondly recalls Seth/Ryan eclipsing any het pairs in popularity at several points during season one and two. This was addressed within the show as well as outside. Addressed as in nearly confirmed.
Funny Aneurysm Moment - Chris Brown's three-episode guest spot in Season 4 as the young fifteen-year-old Kaitlin Cooper's nerd boyfriend is slightly odd to rewatch now.
Growing Back The Beard - Season 4 for bringing back the comedy, focusing back on the core characters, and being without the acting talents of Mischa Barton
It's Popular, Now It Sucks - Kaitlin Lampshades this in season 3 with "Death Cab's playing on The Valley now? Never Listening to them again.", which also doubles as a Fandom Nod, as both the show & Death Cab's fandoms voiced similar complaints when they appeared on The OC.
Narm: Marissa shooting Trey. The creators obviously meant for this to be a dramatic moment, but they inadvertently replaced all the drama with unintentional hilarity by the sudden and out-of-place activation of Imogen Heap music as soon as the bullet hits Trey (as if the bullet had hit the play button on some kind of sound system hidden inside Trey's ribcage), followed by slow-mo as Trey slooowly figures out that he's been shot. Adding to the unintentional hilarity is the fact that the looks on everyone's faces looks less like "Oh my God," and more like "Hey, do you know where that music is coming from?" The clip has to be seen in order for its hilarity to be completely understood: http://www.youtube.com/watch?v=saEzQcayEPM. All together now: Mmmmmmmmmmmm.....whatcha SAAAAA-AAY!
Never Live It Down: People still treat Ryan's wifebeater from early season one as synonymous with his character in season 4.
Who was kind of an intended Scrappy, except not really - his personality and arc was always meant to go the way they did, which for fans and critics alike equalled Epic Fail. If nothing else, the writers certainly acknowledged the viewership's dislike towards the Oliver Arc, if taking jabs at it several times is any indication.
YMMV on that. Some people liked her, and during Season 2 a number of fans probably preferred her to Marissa. Not a buttload of course, but she certainly wasn't universally hated.
Seasonal Rot - Season 3, by many. Some say Season 2 as well. If you watched the show for Mischa Barton or the Ryan/Marissa romance, Season 4.
Spoiled Brat: Marissa, for much of Season 2. Made especially bad when Lindsey, who lives in a tiny bungalow and grew up without a father, shows up to contrast with her. And yet the audience is still meant to have sympathy for Marissa simply because she doesn't like her mother.
And again towards the end of Season 3. In one episode she becomes so bratty that she walks out on her (recently very sympathetic) mother's engagement party, and later Summer finally calls her on her Wangsty nonsense, telling her to grow up. Yet a few scenes later, Ryan tells Volchok to treat her right "because she deserves it".
In fact, you can tell whenever she's being a bitch because she makes Julie Cooper seem sympathetic.
Spotlight-Stealing Squad - When Oliver and Johnny were on the show, it seemed like all of the plots started revolving around them. Even Christmukkah was taken over by Johnny's issues.