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.
Heck, most of this game's soundtrack probably qualifies. It's seriously underrated.
Goddamn Bats: Scorpions move and attack very quickly, and of course inflict poison every time they hit you.
The Metroid-looking enemies, which inflict paralysis on you if you hit them with anything but magic or ranged weapons.
Good Bad Bug: In one room of the second dungeon, one of the enemies will drop an experience orb every single time. pretty soon, your fire fairy will be blasting them for you. Bring enough different kinds of fairies, and this may become a Game Breaker.
That One Level: The Ice Castle. Along with slippery floors, there's also an abundance of hidden spike panels on the floor, hidden "poison fountains" that hurt you instead of heal you, and plaques that poison you if you try to read them. And scorpions. Lots and lots of scorpions.
And while the previous dungeons didn't require any backtracking to previous floors, suddenly you're having to go all the way back to prior floors to hunt down locked doors that you now have the key for.
The Droog Caves, solely because of those randomly generated boulder mazes that are Trial-and-Error Gameplay at its worst.
Even better, the dungeon eventually gives you a Rock Breaker axe, touted as being able to even shatter stone. You can then proceed to bust your way through the boulders in the...oh wait, no you can't, because it doesn't work on them.