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: Metroid Zero Mission
Click here for the YMMV page on the original game.
Anti-Climax Boss: Mecha-Ridley is incredibly easy for a final boss, and can feel like an anti-climax for those unaware that getting 100% of all items triples his difficulty level.
Catharsis Factor: After having to run for your life through a mothership full of Space Pirates and then fighting a boss who's difficult to hit without missing or shooting yourself, the payoff of getting your fully upgraded suit is so, so worth it. Add this to the triumphant Brinstar theme playing again, and you'll feel invincible dashing through the Pirate hordes you once ran from, now slaying them by the dozens.
Metroids are horrifically difficult to fight. Freezing them wears off faster than most enemies, and it's only when they're frozen that they can be damaged, and then only with missiles. If one latches onto you, you can deploy a bomb to make it let go, but drop too many and you'll accidentally bomb jump back into its mouth. They come in groups, so you might freeze one Metroid only to find yourself backed in a corner with another waiting on the other side of its frozen friend, ready to grab you as soon as you kill it. Add this to hazards like lava in some of the rooms, Rinkas that are a nuisance, and thus it only takes one trip from a missed obstacle to fall and become Metroid food. Play on Hard and/or a 15% run, and you'll soon find yourself out of Missiles, desperately re-freezing every Metroid surrounding you and picking off every Rinka in hopes that it drops a missile this time. Did we mention the doors lock so you can't leave the room?
Black Space Pirates, though luckily you are only required to fight two. Even so, they have incredible leaping, aren't shy about purposely running into you, and are immune to every non-beam weapon attack of yours.
That One Boss: Mother Brain. Let us count the ways. First off, you fight her while standing over a pit of lava. And you don't get a platform to stand on. Instead you get just two measly blocks which aren't connected to each other that barely let you take more than two steps on either before you fall. Stand in one spot for too long, and you get targeted by Rinkas that will knock you off, and, just to be petty, are Red Rinkas that aren't guaranteed to give you any health or missiles. Surrounding the area are turrets that fire in random directions, making jumping risky because it's impossible to predict when and where they'll shoot. Even if you made sure to replenish your missiles after destroying all the Zebetite barriers blocking the way to her, you still have to waste a ton of them trying to crack her tank. Also, remember how in the previous games Mother Brain was just a sitting duck who wouldn't fight you as you filled her with missiles? Well, say goodbye to that; now she has a Eye Beam of doom that fires every time you damage her or fall off the blocks. God help you if you try fighting her in a 15% run.
The Chozo Mural is also pretty difficult. You have to shoot it in the glowing orb he's holding whenever it displays a certain symbol, but all the other times it shows you, meaning shooting it then means hurting yourself. You take a lot of damage due to lacking your Power Suit, and can only fire when your pistol is at full charge. Its bouncing is hard to avoid, and the area is pretty small. Plus, it hurls lightning at you, and it's hard to watch for the flashing cues that signal whether it's just a single bolt of lightning or twin bolts that climb up the floor and wall. Defeating it takes a lot of mental juggling.
Ridley in Hard mode is brutal. You have half the health you'd have in Normal mode, but that'll be the least of your troubles. He's much more aggressive, his fireballs deal an obscene amount of energy, and your health drains at a ridiculous speed if he picks you up. Also, staying under his feet doesn't works this time.
Mecha-Ridley becomes one on a 15% or 100% run. His arm slash is highly telegraphed, but it has such a large range that the only practical way to dodge it is to have been out of its way beforehand. Where's out of the way? Up in the air, so you pretty much have to spam Space Jump for the entire fight. His worst attack is his Eye Beams, since they fire in random directions, travel quickly, and can't be blocked through Screw Attack. Shooting his core takes good timing, and you're backed against a wall the entire time. He only seems easy because he's a Glass Cannon, and thus most player don't have to face the worst of his attacks. Fight him for an extended time, and Mecha-Ridley becomes a nightmare.