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.
Game Breaker: The Warrior Ring, which enables you to fire off the first level of your charged attack with just a button tap. You don't get it until the endgame, but it allows you to rip through anything vulnerable to a level one blast from the weapon in question. Note that this does work with the Crystalis sword, and the final boss is vulnerable to this, which is a big part of why it's an Anticlimax Boss. Notable that all of the bosses immune to this all fall under That One Boss.
Goddamned Bats: Bats, birds, harpy-looking things...really, any enemy in the game that can fly. Near the end of the game, you get Toxic butterflies that, upon being killed, release clouds that poison you if you touch them. Once you get the Warrior Ring and armor that has immunity to poison then you're mostly safe, but then the helicopter droids in The Very Definitely Final Dungeon can get you just from the sheer amount of damage you take from them and the fact that they're everywhere.
Porting Disaster: The Game Boy Color port of the game, released a decade later, makes alterations to the game mechanics that make things harder than the original, among other things.
To some, the complete sundering of the plot to something much more basic and boring is even worse.
That One Boss: Pick a member of Draygonian Empire (Well, Kelbesque and Sabera aren't too bad.) The rematch with Mado in Goa Fortress is particularly noteworthy for the same reason Quick Man is noteworthy: he bounces around the room much too quickly to be hit reliably while happily plowing into you in return.