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 appearence, 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.
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. Some would also argue that Jeff Schafer has quite a bit of "chemistry" with Mallory's triplet brothers, particularly Byron.
Kristy/Abby is becoming a pretty prominent pairing in BSC fanfiction.
Nightmare Fuel: Claudia sits for a family in which the husband abuses the kids. After Claudia and her mother rescue the children and their mother, Claudia gets a phone call late at night. It's the abusive father, wailing at her to give him back his children.
In one of the early books, a serial thief was breaking into houses in nearby towns, making calls to prospective targets where he'd instantly hang up if anyone answered. The club starts getting calls where the caller hangs up once they answer, and later in the book, Claudia's babysitting and realizes that there's a prowler outside. It's one of the guys from her class, being a moron, and the calls were from another guy who couldn't work up the courage to ask her out, but it's a pretty Adult Fear-ridden situation.
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 was a huge problem in many predominantly white American communities at the time when the books were first written, but it was definitely taken too far on occasion, to the extent where it almost seemed as though the writers were assuming the reader needed to be informed that not everyone was white.
Snark Bait: Several blogs and online communities are devoted to snarking these books.
Tear Jerker: The death of Louie; Mimi's stroke, deteriorating health and eventual death; and a lot of Abby's autobiography.
Louie's death is particularly-bad because he doesn't simply die of old age. He had to be put down. Readers went through several books seeing Louie the collie as an energetic, happy, beloved family pet and see him slowly become weak and sick to the point where the Watson-Brewers admitted that they basically had to put him to sleep, because they couldn't bear for Louie to be in such pain. There's tons of little moments throughout, too, like David Michael trying to keep a half-blind Louie from going down stairs and getting hit in the eye for his trouble, David Michael opting to sleep downstairs with Louie during his last night home, the funeral...
Jessi's baby brother, Squirt, getting hurt in a car wreck.
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.
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.
For all 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.