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: Milon's Secret Castle
Most Annoying Sound: The sound when you get hurt can be more than a little maddening, especially when it's followed immediately by the Game Over music.
Polished Port: The Game Boy port rectifies a few things to make it a lot less fake difficult. It adds title screen music, an ending cutscene, an inventory screen to check on Milon's stats (accessed by pressing Select) a password system, a bit more Mercy Invincibility when hit and a few bosses have been given new graphics.
Surprisingly Improved Sequel: Do Re Mi Fantasy, but it accomplishes this by axing all of the first game's Metroidvania elements and turning the whole thing into a bog-standard (but beautiful) side-scrolling platformer.
That One Boss: Every boss in the NES version. They take a lot of hits and throw their projectiles randomly. With no Mercy Invincibility, it's easy to have all of Milon's health drained from one battle.