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.
YMMV: The Trap Door
Anti-Climax Boss: The Splund, a large intimidating teleporting monster spends the majority of its appearance terrifying Boni and Drut and being the definition of Nightmare Fuel with it's creepy voice and laugh...then Berk pops him with a needle.
Fridge Logic: Despite Berk's misgivings, nothing that comes out of the Trap Door could possibly be less horiffic than The Thing Upstairs, whom he quite happily serves.
Fridge Brilliance: 'Im Upstairs may be an Eldritch Abomination, but he is civilized and living under his roof offers Berk and his friends safety from the barbaric monsters that live down the Trap Door (and probably around the castle as well). See how all the mosters retreat once the Thing Upstairs yells at Berk. Plus, if you consider the games canon, he even pays Berk for his work.
Nightmare Fuel: Ahem, Creepy, crawly slimy things that stick onto your skin, horrid beasts with tentacles that want to pull you in. Squirmy worms, slugs and snails that lie there in the goo...they'll wait down there forever til they get their hands on YOU!
The screamer ending to the last episode. Especially since anybody who saw the last episode never saw it coming, and it's suggested that the Big Red Angry Thing at least did something rather nasty to the main characters!