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's Return to Dream Land
Breather Boss: Plot-wise, the Metal General, who comes in between the climactic fights against the Grand Doomer and Landia. This was made up for in Extra Mode, where he got the greatest changes, becoming a Sequential Boss with HR-D3. Its appearence pretty much strips him of his Breather boss status in Extra Mode.
Draco in Leather Pants: Magolor. He was a evil guy who tricked Kirby and the others to fight Landia in order to get the Master Crown. Fans often try to excuse his behavior by claiming that Zero or Dark Matter possessed him. Sure Magolor actually becomes Kirby's friend for real in Kirby Dream Collection, but fans gave him this treatment before that even happened.
charged-shot mechanic. Unlike KSS, however, where you had to charge by mashing the D-Pad in various directions, you can charge KRTD's Spark by shaking the Wiimote, which can quickly and easily fully charge the shot (although you can also mash the controller in this game as well) - combined with the fact that the fully charged blast deals ludicrous damage to just about anything, it can take down even the Final Boss in less than a minute. That is, of course, assuming your wrists don't get sore from waggling while dodging attacks.
Spark is even more powerful since, when fully charged, there is a barrier of electricity surrounding him that does decent damage and even protects him from the majority of enemy attacks; either the enemy can't get close enough to attack without getting hurt (and subsequently knocked away), or the projectiles they use are destroyed as soon as they touch the barrier. On top of that, if Spark Kirby is riding on another Kirby's back, the barrier is large enough to protect both of them.
Water deserves mention as well, notably its dash attack. Although it doesn't seem like much at first (Kirby rides a wave forward, with no way to control/stop it until it ends a short distance away), Kirby is literally invincible during the entire attack, meaning you can just coast through anything, from giant boulders to the final bosses' huge lasers. The other attacks aren't bad either; you can shoot a geyser straight up to hit flying bosses, or send blobs of water across the ground with unlimited range. Still, the dash attack is just ridiculous.
Water's Rainbow Rain attack does a good amount of damage and can be chained infinitely, if you're quick enough. Oh, and Kirby is invincible the whole time, and said chaining makes him permanently invincible, and you can also suspend him in midair with this, if you need to hit a floating boss.
Special mention goes to the Leaf ability. Its guard renders you completely intangible to almost all attacks (it won't save you from being sucked into a vortex of pain, for example) so if you're having trouble dodging certain enemy maneuvers, grab Leaf, hold A and forget.
Not to mention the Tornado ability. It makes Kirby completely invincible until he stops spinning, making it easy to avoid damage and punish enemy assaults. Not only that, but it now has a charging attack that is used by shaking the Wiimote while Kirby is spinning. When he stops, the tornado spreads out as it dissipates, causing a ton of constant damage and covering Kirby as he recovers. Needless to say, mastering Tornado is just about the same as mastering Spark, and having at least two Kirbys with both of these abilities will absolutely wreck any boss that stands in their way.
Meta Knight can use his Shuttle Loop from Super Smash Bros. Brawl in this game. It eats at boss HP like crazy and it's very spammable.
Hammer's infamy for this carries on into this game. It's made worse with Dedede, who naturally keeps his Hammer even after using Hammer Throw, meaning he can just spam it as much as he wants.
Ho Yay: Face-to-Face, a returning ability which allows you to heal your teammates immediately after eating a health item by touching them. In game, you kiss your teammates... who are all male. This is made even funnier by the fact that the manual shows a picture of Face-to-Face in action with Kirby and Meta Knight "healing" (coughkissingcough) each other ◊ .
Magnificent Bastard: Magolor, right before he goes completely crazy and goes to take over Popstar after succeeding in acquiring the Master Crown by manipulating Kirby and co. into defeating Landia for him.
Recycled Script / X Meets Y: This game is essentially the gameplay of Kirby 64 The Crystal Shards merged with the plot of Milky Way Wishes. There are 120 energy spheres scattered throughout the game that require skill, paying attention, and/or having the right power to collect them. Two of Kirby's enemies team up with him. As for the plot, Magolor asks you to collect the lost parts of his ship so that he can return home, then he offers to take you there. Once there, you retrieve a dangerously powerful artifact, which Magolor steals from you to rule the galaxy, and you have to stop him.
The Tetris Effect: If one plays through the ability stages enough, they might have the tendency to have to do everything with precision and on the first try.