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.
Sweet Valley Confidential also has broken the base, since it's revelations about the current status of several major characters (most notably Winston and Todd/Jessica) was not what most fans wanted to see happen.
Broken Bird: Elizabeth, in Sweet Valley Confidential. Alone in New York, estranged from her sister and basically making her living sneaking into off-Broadway plays for a third rate magazine that reviews off-Broadway plays, and socially isolated from everyone, even the various one-night stands she has.
Winston as well; in Sweet Valley Confidential, he's gained power and wealth and the ability to get revenge on anyone who ever tormented him. But he's lost everyone who ever cared for him and when Elizabeth rejects him for reformed former tormentor Bruce Putman, he kills himself.
Butt Monkey: Jessica, of all people. Her schemes and stunts consistently blow up in her face, leaving her humiliated (though given her sometimes malicious intent, this is often deserved). Her genuine efforts at improving herself (music lessons, cooking classes) end up the same way. Virtually everyone, even her own parents, appear to blatantly favor Elizabeth over her—not until she runs away, TWICE, does it finally dawn on anyone how troubled she is. When she aces the SATs, instead of congratulating her, everyone, including her own sister, assumes she must have cheated somehow. Speaking of which, Elizabeth herself frequently treats her like something she stepped in. And the few times she genuinely likes a guy enough to have a steady relationship with him, it never works out.
Crack Fic: Many outlandish events occurred, but surely The Magic Christmas, in which a pair of harlequin dolls transport Elizabeth and Jessica to Narnia a fantasy land where they assist twin princes in defeating an evil wizard, must qualify.
Don't forget the Sweet Valley University book in which one of their professors turned out to be a vampire.
Creator's Pet/Designated Hero: Elizabeth Wakefield is purported to be the good twin who is nice to everyone - but, more often than not, she comes off as a meddling, sanctimonious hypocrite. As such, many find her to be even worse than the allegedly bad twin, Jessica.
Draco in Leather Pants: Even the writer was amazed at how much fans preferred the "bad twin" Jessica (who fans saw as more understandable and interesting) to "good girl" Elizabeth (who fans saw as hypocritical and self-righteous).
Ear Worm: "Look right down in the crowded hall, you'll see there's a beauty standing..." You're welcome.