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.