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.
Nightmare Fuel - The enemies are nasty looking, and you spend the last level inside one! (The Sega Genesis version also does away with a good deal of the humor of the arcade version, and changes the mood to be more serious in tone)
The special mention is possibly the Creeper enemies. They don't look scary in Regular Mode, but in First-Person Mode, that's the only way for the player how the Creepers' nasty face actually look like.
And Giant Creeper in Arcade version, dear God, they are Mook Maker! That's a quite escalation from Genesis version because in Genesis version, they are completely harmless.
That One Boss: The weird pink alien thing, specifically the second form. If you go too near it to attack it, a very fast fist comes out of its stomach to punch you. If you try to stay away, it bends over and gobbles you up. The only way to win with somewhat more ease is to use jumping/dashing attacks. The third form after it is considerably easier to evade.