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 Mass Attack
Awesome Music: As per Kirby tradition. What's surprising is that several tracks from Kirby Air Ride were remixed. What's not surprising is that the composer, Shogo Sakai, also worked on the soundtracks for games such as Kirby Squeak Squad, Kirby Air Ride, and Mother 3.
Bizarro Episode: 4-10 and 4-11. In Volcano Valley, a fire-haunted area, we have chasing after a switch taken by astronaut monkeys, buliding a spaceship from a puzzle, Space Is An Ocean, monsters that create black holes, gravity switches in another space station run by said monkeys...
Breather Boss: Lady Ivy, the second world's boss, which is just insultingly easy after the hassle of getting the gold star from Whispy Woods in the previous level. Provided you can figure out that this is pretty much the only boss that dragging makes easier.
Skullord is disappointingly easy, especially after Buzzy Bat...
Scrappy Mechanic: In this game, Kirby has a limited air meter for breathing underwater that must be replenished with air bubbles, which probably doubles as a Shout-Out to Sonic the Hedgehog. Yes, this is as annoying and frustrating as it sounds, and it gets even worse later when you have to swim in toxic water that drains your air meter twice as quickly.
That One Boss: Some players agree that some of this game's bosses are so damn hard it's not even funny. King Eelongo, which is basically flinging Kirbys at underwater moving objects that are spiky almost all over, requires some dexterity. This is one of the more infamous examples.
Interestingly enough, as far as gold stars go, the toughest world boss is the very first one, Whispy Woods, as the timing to attack him without him countering and hurting you in the process is very minuscule. Otherwise, he falls straight into Wake-Up Call Boss territory.
While this is part of 4-9, Buzzy Bat deserves special mention. He's the last miniboss in the game for good reason; the fight deals with breaking his shield of bats and finally hitting him, which instantly destroys the whole shield as you start beating him up. If you take too long, he combines with the shields to form a monstrous cycloptic bat that you can only run from and wait for to break. As you go on, the shields become more complex and starts to add giant, invincible, spiked bats. The final shield has three of them and small openings to actually attack the main bat.
Mr. Shine and Mr. Bright in Strato Patrol EOS. It's a Dual Boss where you have to deal with Mr. Bright firing rather big lasers at you or giant circles of energy at you while Mr. Shine becomes invincible and spins throughout the room. Oh, and if you kill Mr. Bright, Mr. Shine proceeds to revive him. And if you kill Mr. Bright four times, Mr. Shine proceeds to merge with the remnants of Mr. Bright and goes berserk, firing lasers at fast speeds and spinning in a new fashion. Meta Knight later on is also pretty tough, especially when he charges throughout the room. And you fight Nightmare, the Final Boss, immediately afterwards.
Level 3-11: This is more due to Fake Difficulty than anything else, as you're in a giant tree that's swaying back and forth depending on which side you move the Kirbies to, and you have to move quickly to avoid leaning the tree in one direction too long and causing the whole tree to topple over (resulting in an instant death). The hardest part in this level is around the halfway point, where you have to carefully dodge several rows of spike balls followed by a giant spike ball afterwards, which is almost impossible to do while moving all of your Kirbies upward. Fortunately, if you can make it past this sequence without getting hurt, the rest of the level is much easier.
That One Sidequest: Completing all of the list objectives with "Gold Star Champion" standing out as the toughest one due to the fact that you have to beat each level and the bosses without getting hurt to get all of the gold stars.
The Woobie: Kirby, much more than usual in this game.