Dating What Daddy Hates: Ron's family are upset when they learn he's accepted a formal courtship with an anonymous party in Slytherin House. From the hints his suitor has dropped and the gifts he's received Ron guesses that his suitor is an older male pureblood of higher status.
Despair Event Horizon: At the beginning of the story, Harry was suffering so much at Privet Drive that he awoke the Horcrux.
Disproportionate Retribution: Voldemort hates muggles because he was picked on at the orphanage and they killed his baby snake. Voldemort tracked down his childhood tormenter years later and killed her child.
Enforced Cold War: Harry and Sirius after Harry reveals to Sirius that he's Dark. Sirius doesn't support the dark side but he won't betray his Godson.
Even Evil Has Standards: Tom is disgusted by Vernon Dursley. Also Original Character Lethfyren Malfoy, a Mind Healer of shady repute that Voldemort hires (and who Tom calls "a monster") will NOT "do kids." Voldemort himself is strongly against his followers abusing their children though he seemingly has no problems killing them, since he was quite happy to kill at least two children before the events of the story. But then Voldie has a soft spot in his cold black heart for wizarding children-not muggle children.
Everybody Knew Already: Harry, Tom, and Draco aren't as careful as they think. Ron, Blaise and perhaps half of Slytherin house knows that Harry is the Heir but have so far kept quiet.
Evil Mentor: While Tom plays The Corrupter for Harry, Harry acts as Tom's Morality Pet to the point that when they meet Voldemort both Tom and Voldemort are surprised by how much they've changed.
Exact Eavesdropping: This is how Ron learns what Dumbledore's plans for him are and how he learns Harry's secret about "Tom."
Fallen Prince: Draco gets a lot of crap for hanging out with the Boy-Who-Lived and a Weasley especially from his father.
Good Thing You Can Heal: Vernon quickly learns that baby Harry heals very fast and so takes the opportunity to beat him even more.
Heel Realization: After Voldemort tasks Harry and Tom with running distraction duty at Hogwarts by setting the Basilisk loose on the student body they had originally picked the muggle-born Hermione Granger to be one of their first victims, but after a week of stalking Hermione, Harry and Tom realize how brilliant she is and decide to choose a different target. When Harry and Tom give their report at the next Death Eater meeting and explain why they switched targets this even get Voldemort thinking that maybe he was wrong about Muggle-borns.
Non-Lethal Warfare: Voldemort wants the Chamber of Secrets open but (on Tom's insistence that Harry not be forced to kill) Voldemort gives Harry strict orders to use the Basilisk to only petrify and not kill any students.
Training from Hell: Harry when Tom is teaching him magic. Later subverted, Lucius claims that he tortures Draco in order to make him strong and teach him how to be a man. Draco eventually comes to realize that Harry was right—his father does not really have his best interests at heart and is just hurting him for the sake of hurting him.
Transparent Closet: Draco is the trio's resident expert on clothes. Also Harry and Draco share an awkward bro-hug.