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.
Designated Villain: Janine is demonized by Claudia for... it's hard to say, her motives change randomly. Probably because Janine has the unfortunate tendency to be obnoxious (with an IQ of 196, this makes sense)
Alan Gray for Kristy; Cokie Mason for the whole BSC on occasion.
Jessi's Aunt Cecilia qualifies. She is definitely right about how an eleven-year-old, regardless of how responsible she is, should not be left in charge of an eight-year-old and a toddler for a whole weekend. She's wrong, however, in that she seems determined to blame Jessi for the matter, rather than Jessi's parents who left her in charge.
Pamela Harding in the Little Sister spin-off series is meant to be a pint-sized Alpha Bitch, but ends up coming across as this. While she does have some genuinely snotty moments, the main reason Karen started demonizing her was because in her first appearance, at Karen's sleepover party, she didn't want to eat pizza, didn't like The Wizard of Oz, and preferred to sleep in a bed over sleeping on the floor.
Ensemble Darkhorse: Shannon is very popular among members of some snark communities. Amusingly, so is background character Pete Black. Janine also has a following.
Logan is also rather popular; he even got two "Reader's Request" books narrated by him.
Andrew in the Little Sister series.
Fad Dissonance: It's pretty hilarious to read about Claudia's totally weird new friend who wears long dresses and flared jeans, both of which have come back into style since the books were published.
Family-Unfriendly Aesop: Claudia had the nerve to make a friend outside the club, so in retaliation, the other sitters scarfed her snacks, short-sheeted her bed, left mean notes around her room (one even suggesting they leave a blank one, just to mess with her head) and trash-talked her in the club notebook that's supposed to be only sitting-related. It's Claudia who is portrayed as the wrong one and has to apologize in the end.
Fan-Preferred Couple: Claudia's sister Janine and Kristy's older brother Charlie are this for many fans.
"Funny Aneurysm" Moment: In the series finale, the girls make a time capsule to be opened in seven years and each contribute a letter. In Dawn's letter, she talks about flying between California and Connecticut and remarks that maybe by the time the capsule is opened, air travel will have changed somehow. The book was published in 2000.
Hilarious in Hindsight: In Mallory and the Mystery Diary, published in 1989, Mallory complains that it feels like she's been 11 for a decade. Cut to 1999, when the books are still being published and poor Mal is still 11...
Ho Yay/Les Yay: Lesbian subtext is a popular topic of discussion among fans.
Kristy/Abby is becoming a pretty prominent pairing in BSC fanfiction.
National Stereotypes: The Australian family in Kristy and the Secret of Susan has the surname Hobart (the capital of Tasmania), eat vegemite, use slang that no one in Australia has used for ages (like "brekky" and "funny as a funeral"), and have to endure tons of Crocodile Dundee references.
It's probably easier to catch the Loch Ness Monster than it is to find fans of Karen Brewer.
Jessi also gets a fair amount of abuse from fans due to her lack of characterisation beyond being the Token Minority and the fact that the entire club seems obsessed with her being black. Somewhat justified given that racism would have been a huge problem in the setting (a small, predominantly white American community in the eighties): however, the series went way overboard to the point where Being Black became pretty much Jessi's defining character trait.
Snark Bait: Several blogs and online communities are devoted to snarking these books.
Straw Feminist: In "Little Miss Stoneybrook... and Dawn", Jessi and Mallory make their disdain of beauty pageants known, saying they make little girls think just being cute is important. Never mind that the whole group puts Stacey and Claudia on a pedestal for being stylish and pretty, and Mallory herself often whines about how much she hates that her parents don't let her lose her braces and glasses or dress glamorously...
Unfortunate Implications: Every description of Mallory and Jessi includes their similarities and differences. The problem? One of the girls' "differences" is that Mallory is white and Jessi is black. No matter who is narrating, they use this comparison EVERY SINGLE BOOK.
Oddly enough, the Little Sister and Kids in Ms. Coleman's Class spin-offs handle this much better. Ms. Coleman is black, as are several of the kids in her class, but you almost have to look at the pictures to realize this, as the narrator (usually Karen) really doesn't mention it.
Kristy and the Secret of Susan is notable for its ridiculous Hollywood Autism and how the disorder is addressed. Martin gave her every single symptom imaginable (this does not happen in reality) and portrays her as the stereotypical savant with all sorts of impossible abilities. Kristy completely oversteps her bounds and tries to shame Susan's parents about sending her to a boarding school (never mind that said school had very good professional programs for her) and tries to force Susan to make friends (VERY difficult even for mildly autistic people) by introducing her to the neighborhood children, who treat her like a freak and coerce her into playing memory games.
Unpopular Popular Character: Janine is an unpopular smart girl who has very few friends. Her sister Claudia can't stand her most of the time, and also Claudia's friends (at least in early books) tend to think she's an annoying Insufferable Genius and usually try to avoid her. However fans love her, and she's by far one of the most popular minor characters.
Jenny Prezzioso, to some extent. Every single time she's mentioned, the sitters say something mean about her or talk about how horrible she is. To be fair, Jenny is a Spoiled Brat but it's mostly due to the way her mother raised her and then again, she's four years old. For this reason, Jenny gets a lot of sympathy from fans who think she's not nearly as bad as the sitters think.
Values Dissonance: Kristy and the Secret of Susan: while it may have seemed progressive at the time, the attitude towards autism in the book has not aged well.
The Woobie: Just about all of the major characters get their fair share of woobification.
Mary Anne in the early books was shy, prone to tears and severely overprotected by her dad.
Stacey, due to her sad backstory, being frequently hospitalized, and dealing with her parents' divorce.
Kristy, due to her dad not being around, amongst some other family problems. Taken Up to Eleven in the movie.
Abby and Anna, due to their father's death; their mother also, albeit to a lesser extent.
Although Shannon doesn't get a lot of mention, it's clear that her home life isn't exactly peachy.
As much as many fans dislike her, it's difficult not to feel sorry for Dawn due to the fact that she can never have her family in one place.
Claudia has her moments, such as on the occasions when she feels misunderstood by her family (and isn't just being a paranoid Bratty Teenage Daughter for once) and when she can't get the grades she wants, no matter how hard she tries.
Nicky Pike has his moments, mainly when the triplets are being mean to him.
Jerkass Woobie: Karen Brewer if you really consider what she's had to live with. Her parents divorced and remarried within a year and she was apparently expected to adjust pronto despite being six years old at the time. In the earlier books at least, she spend only two weekends out of every month with her father, and he still insists on leaving her with a baby-sitter for most of that time. Then Watson and Elizabeth dropped a "surprise! We adopted a baby!" bomb on the whole family. Her behavior may be inexcusable at times, but it's hardly surprising.