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 Epic Yarn
Best Boss Ever: Squashini, with his showiness and creative attacks and awesome music plus being a well-done slot-machine boss, and Yin Yarn, with being a Final Exam Boss done right. Especially his second form.
Freud Was Right: Nintendo sent a mail advertising this game, but most of its recipients were looking at the placement of Kirby's yarn whip instead...
Game Breaker: Having a second player means having effectively infinite ammo, the ability to Double Jump via standing on your partner's head, and having someone watch your back while you're doing something else.
Surprise Difficulty: Any review of the game will mention that Kirby is invincible – getting hit only loses beads. In the (timed) minigames, getting hit loses beads and takes five seconds off of the clock. In some of the later stages, the time margin is so strict you have to complete the entire level flawlessly to succeed.
This is just as true for getting a gold medal on many of the later story levels. There is a serious paucity of beads, and the bead requirements become much more strict. Several levels will essentially require you to restart if you make a mistake.
As a result, this seemingly simple, easy and absolutely adorable game can get very, very frustrating if you try to go for 100% completion, so much so that it can almost count as a Mood Whiplash.