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.
Breather Level: Octarina's dance levels during Calypso Beach. Also, anything involving the crash-cart.
Fridge Brilliance: There's playable characters who use Percussion instruments and String Insrtuments, but why no Wind instruments? Because you're already playing as him most of the time.
Game Breaker: The ability to go back and replay levels, just to gain extra hearts. Not a major one, but it can be exploited to the player's advantage, particularly if you're really struggling in the later levels. Granted, it will only allow you to collect up to 8 hearts, but that's enough to see you through some of the more difficult levels.
Narm: Its tough to catch, but in the sequence before Discordred's boss fight, Tempo goes up to the side of the ship monster, and the face on the back looks at him. The face on the rear end of the ship is reflected, so while one eye on one end of the face looks at tempo, the other one looks the opposite way on the edge of the screen, effectively making an angry derp face.
Squick: Otamama's attacks. It forces out its eggs (sometimes its fully formed babies) in hopes of taking you down.
And then you proceed to smack its babies back at its rear...
That One Boss: Buzzooka is this due to it being hard to hit the enemies he fires out,the canyon section involving the use of the D-pad and being a long battle. The fact that it was the boss in the Demo doesn't help nor the fact that the Final Boss is nasty in this regard as well.
That One Level: Clockwork Trial. The music's speed shifts erratically, and it occurs during action-intensive parts, meaning you'll have to adjust while already playing to the beat. Doing it on fast makes it ridiculous.