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.
Good Bad Bugs: For some Copy Abilities (namely Sword, Parasol, Hammer and UFO), it is possible to keep them forever by discarding the ability while pausing the game and exiting a level. The game will treat Kirby as if he has no power, while he still has it. This means getting hit will not cause Kirby to lose the ability (discarding the ability or dying will, however, and reswallowing the star doesn't restore the ability), and UFO Kirby can be used in every level and in the hub areas.
Mr. Shine and Mr. Bright also qualify in the eyes of some players, since it can often be difficult to deal with the one on the ground and the one in the air at the same time.
Meta Knight may very well be the hardest boss in the game - for starters, the Sword ability is forced onto the player, preventing any alternatives from being used (and unlike Kirby Super Star, picking up the Sword is required to even start the fight). Secondly, Meta Knight moves very erratically, making contact damage almost guaranteed. Finally, he is invincible unless attacked at specific times (namely while in the air or from behind).
Visual Effects of Awesome: The rotating parallax towers in Butter Building. Running around them even affects nearby enemies. No wonder people felt disappointed when this set-piece was dropped in the remake in favor of a much lamer mist effect.