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: Kirby and The Amazing Mirror
Awesome Music: The themes for Mustard Mountain, Radish Ruins, and Candy Constellation. Also Dark Mind Form 2's theme.
Goddamned Bats: The Bronto Burts. In addition to them, there's the Sword and Heavy Knights (they move fast), Leap (they'll kiss you if you're not careful in the air), and Roly-Poly (He rolls down slopes and bumps into you if you're not careful). Also the Batty enemy.
Droppy, who steals your ability and turns into the enemy you got it from. Even worse, if it steals a boss-only ability like Hammer, Throw, Smash, or Master, it instead turns red, runs off with it, and attempts to commit suicide at the first opportunity (either by running off a cliff or running into you) in order to prevent you from regaining the ability!
That One Boss: Mega Titan. His impenetrable armor (except against electric attacks) and fast attacks make for a frustrating fight.
Phan Phan qualifies as That One Mid-boss. Between his pattern being randomized, moving insanely fast, and his attacks coming out quickly with little warning, he is by far the most annoying miniboss to fight.