These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Ensemble Darkhorse: Pretty much the entire cast depending on who you ask. The characters are well-written enough and well-acted enough (and significantly, diverse enough) that that you could put almost any of them in this field. But special mention has to go to Glen Coco, a character who we do not see, do not hear, and only hear one line about, and is still this trope for some.
Glen Coco? Four for you Glen Coco! You go Glen Coco.
According to the special features, the Mathletes were the favourite characters of the cast and crew.
Regina George herself has been known to be the show stealer for many people too, as have Gretchen and Karen.
Follow the Leader: This movie has been called a PG-13 version of Heathers, and not without justification.
Harsher in Hindsight: Regina's line "I couldn't invite her to my party, I mean, right, she was a lesbian!" to Cady became this trope when Lindsay Lohan started a romantic relationship with a woman.
For an added bonus, take a look at the list of songs during the credits. One of them is performed by Samantha Ronson, Lohan's on-and-off girlfriend.
The fact that the film was toned down to PG-13 after Lindsay Lohan was cast, due to her being too "family-friendly" to star in an R-rated film.
Before playing Gretchen Wieners, Lacey Chabert was the voice of Eliza Thornberry, who traveled around exotic lands with her parents like Cady.
Though it also counts as a "Funny Aneurysm" Moment depending on your view, the movie follows Cady as she falls in with the wrong crowd and goes from a mild-mannered nice girl into a party-happy tempermental bitch...
Lohan's constant arrests and trips to rehab afterwards have led to the widespread idea that Regina got the last laugh on Cady in the end.
Cady sees two girls making out at the Halloween party and looks very dismayed.
Gretchen. Regina comments that Gretchen isn't that pretty, implying that Gretchen is only popular for being rich, not for her looks like Regina and Karen and Cady. The problem? She's played by Lacey Chabert.
Janis, who is played by Lizzy Caplan◊. To be fair, the producers were reluctant to cast Lizzy as Janis because they felt she was too pretty, so they instead gave her an "alternative" look and showed that her lack of popularity was due to her fashion sense and past events instead of her being ugly.
The scene itself makes Regina something of a Memetic Badass in-universe.
Misaimed Fandom: Oddly, this movie seems to be most popular with the same kinds of people that it mercilessly makes fun of.
Moral Event Horizon: "Put it in the book." It's where Cady herself admits she's begun to act like a bitch; though at the time, she tells herself she's just pretending.
Older Than They Think: The often quoted line "It's like you've got ESPN or something" is usually attributed to this film, but that exact line actually came from an episode of Will And Grace (the line was said by Jack) that aired 3 years before this film came out.
It was probably first said before that; it's a malapropism probably almost as old as the channel itself.
Sequelitis: Apart from the element of Jo creating a group of Anti-Plastics and the climactic football game, the film is pretty much a remake of the first film, without the laughs. Also Jo's friends turning against her is extremely forced.
Tina Fey repeatedly turned down offers to make the sequel. Now that the sequel was done without her, she regrets turning down the offer.
Unintentionally Sympathetic: Mandi's step-brother is really callous towards her. It's not exactly undeserved but since she seems to like him, Taylor can come across as a bit of a jerk and arguably even as a contributing factor to Mandi's bitchiness.
