This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Ginger Snaps
The werewolf howls, they do not sound much like a natural wolf howl, they sound like a low long drone.
In this movie there is the idea that once you turn into a werewolf, there's no going back.
Doubled by the fact that the generally-accepted remedy for lycanthrophy, wolfsbane (although they use "monkshood" which is of the same genus) isn't a cure, it's a retardant that slows down the transformation, but ultimately doesn't stop it.
The Beast in Bailey Downs encounter, just how fast it runs and how strong it is. Just imagine, running through the dark woods, with that thing running in howling after you and it's fast enough to catch up to you.
Sam's death. Basically he was torn up a little bit, and was being kept alive by the werewolf Ginger, but was bleeding heavily. Ginger stops her attack when Brigitte shows up and she begins to give in to her developing wolf side and drink up some of Sam's blood. When Brigitte vomits it out and comes to her senses, the Ginger-Wolf bites Sam's neck, killing him.
The moment when Brigitte sees that Ginger is starting to grow a tail.