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.
Fridge Horror: Shiver Star appears to be Earth (note the continents and moon), but it's completely covered in ice. And there are (man-made) robots who live in a (man-made) factory, but humans are nowhere to be seen...
Maybe it's just the Earth during Winter season. This coupled with the fact there's a toy level strongly suggests it's Christmas time.
The state of matters at Ripple Star really drives home the need to stop Dark Matter. It's seen in the cinematic to be slowly being corroded, and indeed each stage looks worse and worse for wear. The horror comes in realizing that this is what Dark Matter plans to do with the rest of the universe.
Game Breaker: For the few useless powers, this game has a plethora of absolutely devastating powers that will allow you to mow through enemies and bosses with ease.
Cutter + Bomb gives Kirby explosive ninja star throwing power. It's extremely rapid fire and hits almost instantly. Most bosses go down very fast.
Fire + Fire basically turns Kirby into a comet that allows Kirby to soar for long distances, destroying everything in his path. He can also fly for much longer after Kirby has used his breath, allowing him to soar for great distances past whole levels. You can't control your flight as the comet, but that's rarely a concern.
Ice + Ice turns Kirby into an almost indestructible snowball. All enemies become absorbed in his path. The player doesn't even have to do anything most of the time. Just grab some popcorn and enjoy the mayhem.
Rock + Cutter turns Kirby into a stone form of one of his past companions. In stone form, Kirby is invincible, plowing through most enemies while at worst losing his stone skin if he hits a Gordo or other similar spiked item (without losing health or the power). Now, just mash B until you get Pitch (the small bird), who can fly almost indefinitely, and you'll pass through most levels easily. As a bonus, Kirby won't be affected by water currents in this stone form, which helps immensely in a certainwater level.
Fire + Cutter. In addition to looking really cool, this BFS can also be held up and thrown. Helps a lot when fighting Acro and HR-H/E.
Spark + Cutter gives you a double bladed lightsaber. What it lacks in the versatility of the fire sword, it makes up for in speed.
Not only does Electric + Rock look cool, but you can hit enemies from every angle if you let the rock bounce around on its electric cord. You can casually walk through some levels with this combo.
Rock + Needle. It allows you to fire a drill, that destroys every enemy in its path (except invincible ones). Also, you can charge through people while holding on to it, making you an unstoppable projectile of doom. Also, it button mashed while in the air, and if a direction (horizontal) is held down, you can float insane distances, because activating the drill stops you from falling. Platforming and fighting help all in one.
Ice + Electric. It turns Kirby into a refrigerator and pops out three food items. And it can be used in boss fights. That's right; this power lets you heal during boss fights!
Electric + Electric surrounds Kirby with a forcefield that instantly zaps any enemy that touches it.
Fire + Bomb. Kirby jumps, spins, and shoots fireworks all around him. Usable three times before you have to land, and gets bigger with each use. You keep all of your momentum and mobility when attacking, making it great for moving through levels quickly.
Bomb + Bomb equals homing missiles.
In short, with experimentation, even newbie players will stumble upon at least one combo that makes the game a breeze. As a result, the biggest challenges are found in 100% Completion, where you are often required to use a particular ability — not necessarily an effective one — to find a Shard. Just a warning, though: Do not expect to use these for the Final Boss, Miracle Matter, who holds the distinction of being the only boss post-Dream Land where copy abilities are next to useless.
Stage 6-3's music. Jazz has never sounded so "oh god something is coming something bad is going to happen HURRY HURRY".
The bad ending, when you realize the Fairy Princess/Queen is glaring at you.
Said glare is accompanied by a Last Note Nightmare, made even more effective by the rest of the ending playing out as if everything was fine beforehand.
Nightmare Retardant: When fighting the True Final Boss, his vulnerable locations are his blood-red eye (the bleeds more when you shoot it), its halo of power, and... its cactus-butt that farts at you?
That One Boss: Final Boss Miracle Matter, for most, is strong for Final Boss standards. For aKirby game. Particularly his bomb form, which fills the screen with painful bubbles, and his spark form, which attacks with ricocheting, screen-wrapping lasers.
That One Sidequest: The Crystal Shard in level 4-2 that requires you to carry the Dynamite powerup all the way to the end of the level (because nothing in 4-2 itself will give you that particular combination) is widely regarded to be most annoying (or close to it) Shard in the game to get.