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: "The Cloaked Man" from season two (actually Vice Principal Stern), is an arrogant and genocidal wizard out to wipe out all vampires, knowing that not all are evil, and that some are even his students. He starts his operations by summoning the breath of death, a mist to drive vampires to kill each other, before the human Ethan foils his plan. Upon finding out Ethan has seer powers, and after being exposed, The Cloaked Man casts a spell on Ethan to cause him to hallucinate everyone being dead, just to make sure Ethan doesn't cross him. The Cloaked Man also gets his hands on the Lucifactor, a energy draining weapon to kill vampires, and goes on a killing spree before planning on using the Caller beacon to wipe out vampires en masse. During the climax, he tries to distract Ethan by casting a spell that makes Ethan's friend Benny attack him.
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.