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.
Damsel Scrappy: You could make a drinking game out of the multiple times Ariana gets kidnapped and needs to be rescued.
Fridge Horror: In this universe, Snape's loving parents were killed by werewolves. Sirius knows this; he was, in fact, right there when Snape found out. Remember that whole "try to get Remus to kill Snape" prank? This fic somehow made Sirius look even worse for doing that, though the writer doesn't seem to have realized it.
Black Hole Sue: Everything is about her. All the characters seem to think about is how *insert terrible thing here* could affect Ariana. Hell, Voldemort even gives up on Harry so he can attack her instead!
Fixer Sue: Sirius Black is proven innocent with her help, and she convinces both Ron and Harry to tell Hermione and Ginny, respectively, that they love them. Later she flat-out tells Harry to propose to Ginny, and he does.
Jerk Sue: Has shades of this, especially concerning her rival, Maria. Ariana claims Maria hates her guts, she's a bitch, etc., and yes, Maria does hate her, but most of their conflicts are either started or made worse by Ariana herself. Also, Maria's abuse is mostly verbal, whereas Ariana constantly threatens to attack Maria with her powers, which she knows can kill someone. When fisticuffs break out, guess who always starts it?
Purity Sue: Wise beyond her years, stunningly beautiful at age twelve, has super-rare powers, everyone she meets either loves her (the good guys/people the author likes) or wants to rape her (villains/people the author doesn't like).
Thirty Sue Pile Up: Ariana, her friends, Maria and her friends, every damned Canon Character Ariana comes into contact with...
Ron the Death Eater: Ron himself is a bit dumber, but mostly spared. No, Draco gets hit with this worst, being turned from a relatively ineffectual jerk who eventually realized he wasn't cut out for the big leagues, to a pedophilic near-rapist.
Squick: Draco Malfoy lusting after and attempting to molest/rape a first-year.
The fact that she has a consensual relationship with fifteen-year-old Neville is pretty messed-up too.
Strangled by the Red String: Ariana just sort of tells Ron that he's in love with Hermione (and blackmails him into telling her), even though the two had no real romantic interaction prior to this. (They went to a dance together, but that's basically a throwaway line.) Harry's attraction to Ginny has slightly more built-up, but still starts mostly just with Harry making vaguely lecherous comments about her.
They Just Didn't Care: At one point in the second story, Ariana outright admits she thinks she's better at everything than Maria, and that's the cause of Maria's jealousy. And yet the Author tries to claim she's not a Mary Sue...
Villain Decay: Voldemort goes from being, well,Voldemort to the equivalent of a silly mustache-twirler in this fic.
Unintentionally Unsympathetic: Ariana's mother, Tabitha. In the spin-off oneshot, Tabby Cat, all she does is visit Sirius in Azkaban to rant at him. The reader is allegedly supposed to sympathize with her, but the way she complains about how being his sister has made her life a "living hell" even though he has it much worse than she does doesn't make her too pitiable. It doesn't help that her choice of words implies she abandoned him without question the moment she found out he might have been responsible for the Potters', Pettigrew's, and the Muggles' deaths, never mind that he was her brother she had apparently loved, making her look like a Fair Weather Friend.
Worse, she had basically disowned him before that: they were best friends as children, but when he went off to Hogwarts and she turned out to be a Squib she basically cut him out of her life. Sirius himself tries to put a sympathetic spin on his, saying she was "hurt" to realize she had no magic, but the fact is she spent the last decade of her life blaming her innocent brother for her situation.