Awesome Music: The themes for Mustard Mountain, Radish Ruins, and Candy Constellation. Also, Dark Mind's second form theme.
Olive Ocean as well.
Contested Sequel: This game has quite a bit of this going on due to the Metroidvania aspect of the game. While plenty of fans enjoy this take on a Kirby game, there are some fans who have criticized it for being overblown, confusing, poorly-designed and not meshing well the Kirby formula.
Scrappy Mechanic: Some spots in the game require you to hold on to a specific ability until you reach a puzzle that requires it. If you take the wrong path, you'll have to return to the hub and retread the path, and if you lose your ability along the way, you'll have to find it again.
One-way doors, which can make exploration and backtracking more tedious as you may have to leave the area and retrace your steps if you take the wrong door and it happens to be one-way.
Phan Phan qualifies as That One Mid-boss. Between his pattern being randomized, moving insanely fast, and his attacks coming out quickly with little warning, he is by far the most annoying miniboss to fight.
Box Boxer is also very annoying with his constant grabs, throws, and stunlocks.