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.
Complete Monster: At first glance, Vice Principal Stern is just a Sadist Teacher, but that's just a cover for the genocidal wizard he is. Stern arrives in the town of Whitechapple to kill all vampires, even knowing that not all are evil, some of which are his students. He starts his plan by summoning the Breath of Death, a mist that drives vampires to kill each other, but that plan falls flat. Later he realizes that his non-vampire student Ethan Morgan is a seer and potential threat to his plans, so he attempts to kill Ethan, before later tormenting him with a hallucinations of his worst fear (of everyone being dead). He later steals a energy draining weapon called the Lucifractor and goes on a killing spree on the vampire council. He attempts to attach the Lucifractor to the Caller, a beacon of sorts for vampires, to kill vampires everywhere, and when Ethan tries to stop him, Stern casts a spell on Ethan's friend Benny to turn him evil.
Moral Event Horizon: Jesse crossed it when he bites Ethan to force Sarah to give up her last chance of humanity. Although he claims to have done much worse.
Nightmare Fuel - The first episode gave us the zombie animals...especially the dog
The episode Guys and Dolls has that doll that's turned into a 30 year+ woman obsessing (ala pedophilia) over the main character's little sister. Not to mention her falling apart and getting damaged and draining the life force out of humans (and vampires) and turning them into dolls.
Later at the end of the episode when Debbie Dazzle has been turned back into a doll, while all the characters are dancing, she turns her head and winks.
Possessed Rory in "Die Pod" (the special effects worked a little too well), and possessed Sarah in "Three Geeks and a Demon".
Evil Sarah in "Independence Daze", on a psychological rather than visual level. The idea of sweet, lovable Sarah turning evil is terrifying.
So Bad, It's Good: Mainly because it tries too hard to be dark, leading to a lot of Narm. That, and the Windows Movie Maker quality special effects.
Shifting towards just plain good in season 2, with some genuinely witty dialogue, subtle characterization, much improved acting across the board, and an even more post-modern, satirical sense of humor. There's still Narm, but it works and adds to the fun. So of course it had to die.