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: Yoshi's Island
Big Lipped Alligator Moment: Upon clearing World 2 in Yoshi's Island DS, you're treated to a Cut Scene where Yoshi attempts to carry all three (at the time) babies at once. Unlike the other cinematics, this does not move the plot one inch.
Breather Boss: Quite common, with Salvo the Slime in the original, Cloud N Candy in Yoshi's Story and Gilbert the Gooey in Yoshi's Island DS. Funnily, two of the three mentioned examples are the first world's end boss, and usually come after a somewhat more difficult first boss in said world. Marching Milde, as mentioned in the Anticlimax Boss section, is also one of these.
Contested Sequel: Pretty much every game that attempts to follow up on the original, though Yoshi's Island DS was the most well-liked of them.
First Installment Wins: The original Yoshi's island is still much more liked than Yoshi's story and Yoshi's island DS. So Yoshi's New island had been presented like a successor to the original.
Game Breaker: Collectible items like those Bullet Watermelons. It's very easy to use them to just blast spike pits to pieces, since the seeds can destroy thorns on contact and you get an absolute ton of them from just one melon.
Bandits can easily be these, as they run away with Baby Mario quickly and can jump onto small platforms through hordes of enemies with no problem. God forbid one gets caught behind some death spikes with the baby....
The literal bats found in cave levels. Especially in Yoshi's Island DS when the bats just keep re-spawning.
Good Bad Bugs: A programming oddity causes one of the introductions screens right when starting a level (that shows the level number and name) to stay on the screen longer than it usually does. Fittingly enough, it was The Very Loooooooong Cave. However, the GBA version fixed this, so it is only in the original version.
Mondegreen: The end-of-level music in Yoshi's Story.
Most Annoying Sound: Guess. Done on purpose, because play-testers would ignore Baby Luigi at their own peril with a less ear-piercing wail.
The squeaking noise when you slip on ice (or when rats are nearby) can count as well.
Most Wonderful Sound: The sound of getting all the red coins. Try getting that last red coin when also you're one away from a 1-Up.