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.
In book twelve, the moment when Kitty finally lays eyes on the spirit of the first Regina Luporum, and later when she gets the police sketch artist to draw a picture for her.
David's phone call home to his parents at the end of "Il Est Né", mirrored by Kitty's call to her own parents.
Ho Yay: When Kitty enters the Band of Tiamat's suite, she discovers "the place smelled thickly of sex. As if—what else were a bunch of hunky men supposed to do when they weren't onstage?" With the band being all male and the female assistants for the show not living with them (or being lycanthropes), this rather strongly implies at least bisexuality among the band. You know, the Religion of Evil promoting chaos and blood sacrifice.
Moral Event Horizon: Carl starts out as just violent and possessive, but moves up to sexual assault, rape, and eventually torture. And murder, in the case of poor Jenny.
Also, as of book 7 Kitty herself has crossed her own personal line, having turned someone into a lycanthrope against their will, something she swore she'd never do. Considering the fact she came close several times to losing herself in the Wolf and even harming her friends and allies, she maybe rightto worry.
Well, she was trying to kill him and vice versa and she doesn't regret trying to do that. Later, her Wolf side instinctively wanted to protect him from a vampire, but she suppressed the urge because he didn't deserve it. And in an earlier book she offered to turn a family member, so her own personal line is a bit blurry. A bit of Angst was natural and expected in the situation in book 7, but it's nowhere near a What the Hell, Hero? moment.
Never said it was. The implication was that having crossed the line once, albeit for a very justifiable reason, if in order to stop the forces of darkness/chaos Kitty is forced to keep doing such things, eventually she may fully cross the line and have a true What the Hell, Hero? moment in the future. She probably won't do so since she's the Hero, but it seems likely the author will address the issue and show some Angst regarding it.
Romantic Plot Tumor: Kitty Takes a Holiday is ostensibly about fighting a curse, but the major plot thread running through the book is Kitty's Rescue Romance with Ben It even displaces the main plot by the end- The Man Behind the Man turns out to be an Anti-Climax Boss, and the final "battle" is essentially Kitty and Ben vs. Ben's issues with his lycanthropy.
There is a bit more to it than that, since facing said Anti-Climax Boss and obtaining info about the skinwalker is essential to clearing Cormac's name, the face-off with the skinwalker is actually quite intense and climactic, and helping Ben deal with his lycanthropy, and not be Driven to Suicide is a dramatic and well-written character arc in its own right. But the romance does take up more room in the plot than it probably should.
Tear Jerker: Right from the beginning the death of T.J. is this, as is Jenny in book 4, but the slew of deaths in House of Horrors ramp it up to eleven. Especially traumatic are the deaths of Ariel, Gemma, and Jeffrey Miles.
Kitty's meeting with Ned, and realizing he knew Shakespeare, so that she in turn had a connection to him and could ask anything about the life and times of one of her literary heroes, comes across as a combination of this and Crowning Moment of Heartwarming—she gets quite choked up and literally ends up in tears herself.
Kitty saying goodbye to Rick at the end of Rocks the House.
The Untwist: David turning out not to be the serial killer in "Il Est Né" was pretty much this, since the Coincidental Broadcast and the set-up with his uncontrolled Changes and lack of memory of his actions was rather too obvious; if he really had turned out to be the killer, that would have been more surprising (and out-of-character for Vaughn, since Darker and Edgier moments aside, she seems to not be not quite that cruel to nice, good, or heroic characters—at least not until book 7). Who the killer really did turn out to be was a nice twist, though.
The Woobie: Estelle in book 1, Jenny in book 4. The latter is especially poignant as a What Could Have Been for Kitty herself. Karin in book 10 also counts. Also, David in "Il Est Né."