- Anti-Climax Boss: Mother Brain is a stationary target whose only methods of attack are her turret cannons and rinkas. It's a tricky fight, but if you have enough missiles after you destroy the Zebetites, it's far from insurmountable.
- Awesome Music: For an NES game, Kraid's Lair is quite possibly one of the most eerily haunting 8-bit themes of any video game track in history.
- Breather Boss: Compared to how hard the rest of the game is, especially the level where he resides, Ridley is a ridiculously easy boss fight. He just hops up and down in place while spitting a bunch of fireballs at you. You can just freeze his fireballs in place or just stick right next to the ledge he's on. He can't attack while his fireballs are frozen, so he's a sitting duck for your Ice Beam, Missiles and Screw Attack. This is in stark contrast to virtually every appearance he makes after this game.
- Demonic Spiders:
- Metroids. They frequently appear in swarms, and can move fast and can grab you from any direction, which makes them very tricky to dodge. And if they latch onto you, they can quickly drain all your health in mere seconds, and the only way to break free is to spam morph ball bombs (and even then, it can leave you open to be grabbed by another swarming Metroid nearby—no Mook Chivalry here—or even worse, it can accidentally knock you right back into the Metroid's maw). Oh, and they're immune to all of your weapons except for freezing them with the Ice Beam and pummeling them with missiles—even the Wave Beam and Screw Attack just knocks them back for a second. And if you don't have the Ice Beam? Well...
- Reos (the bird-like bugs in Brinstar and Norfair) are another ruthless enemy to deal with, and they're quite common. Until you get the Long Beam, you're a sitting duck for their fast aerial attacks, and they swoop in an arc (and in a game where you can't shoot at an angle), so it's very hard to get a good shot at them without putting yourself in their line of attack. And even when they do descend, they tend to hover just barely above or below your line of fire, and they take four shots to take down combined with a small hitbox. Jumping over them isn't recommended either, because they have a nasty habit of ascending just when you try to go past them. Sometimes they even descend too low for you to dodge them by using the Morph Ball. What makes them even more irritating is that in some tunnels, they pop up in groups, or alongside lots of other local enemies, just to make your life miserable.
- Just forget about Mother Brain, because Rinkas are the true final boss of Metroid. You know what you're in for as soon as you enter the second room of Tourian and immediately take unavoidable damage from Rinkas that spawn near the doorway, and they only get worse when you reach Mother Brain's chamber. There's always more than one Rinka on screen at a time, they respawn endlessly, and they aim directly at Samus's current position when they spawn, preventing her from standing still. Even with the Varia Suit equipped, they shave off a considerable amount of damage with each hit. Worse, they cause knockback, which is all that it takes to knock Samus off the tiny platform that would let her shoot Mother Brain; if Samus gets knocked into the pit of lava in front of Mother Brain, she is trapped there until she can freeze a Rinka and use it as a platform, but she's just as likely to have another Rinka immediately knock her back into the lava as soon as she's out.
- Fair for Its Day: The way Samus reveals herself to be a women, by wearing either a leotard or a bikini, depending how quickly you complete the game, may come off as crude today. But when the game was first released, learning that you actually played a woman who had to fend off Space Pirates and parasitic aliens all by herself was pretty huge back then.
- Good Bad Bugs:
- Inputting the password "ENGAGE RIDLEY MOTHER FUCKER". The results are hilarious, if suicidal. Note: Do not attempt to do this on a 3DS, though - It's infamous for seriously messing with it should you attempt to do so. The code can also crash NES emulators if you use it on the re-release version of the game, though it simply exits out of the game with little more than an error message in the Nintendo Switch Online NES library.
- The password system is designed to handle a lot more data than necessary. Typically, only the bosses beaten, power-ups collected, number of missiles, play time, and starting location are needed. However, other memory addresses can be manipulated as long as the password checksum adds up. That's how passwords like the one above alter the functionality of the game.
- The Door Glitch that allows Samus to phase through walls and execute many a Dungeon Bypass.
- It Was His Sled: Suffice to say, there's a reason Samus is the Trope Namer for a certain trope.
- Memetic Mutation:
- The "Justin Bailey" passwordnote spawns Samus in her pink leotard with almost all her power-ups. Theories have abounded about who Justin Bailey is or it's a reference to a so-called Australian phrase "just in a bailey (swimsuit)", but it's been confirmed the password is a coincidencenote .
- A couple of other entertaining coincidental passwords also exist and have become memetic. Besides the infamous "ENGAGE RIDLEY MOTHER FUCKER" password mentioned above, there's "MOTHER BRAIN? FUCKIN TOASST", which actually creates a valid file where Mother Brain is already dead. The side effect is that it sets your playtime to over three hundred years, ensuring that you will be stuck with the worst ending. Additionally, there is the "ODDISH TAUROS MEWTWO VULPIX" password, which is somehow also valid.
- Narm: Some of the sound effects in the Famicom Disk System version can be this - most prominently the Screw Attack sounding like an electronic fart and Kraid/Ridley sounding like they say "OW!" whenever they get hit.
- Nightmare Retardant: In contrast to his fearsome redesign from Super Metroid and on, Ridley's cartoon alien design looks downright goofy in this game. Same with Kraid's impish stature compared to his titanic redesign and, to a lesser extent, Mother Brain's less detailed appearance (if thanks to Graphics-Induced Super-Deformed) compared to later games.
- Quicksand Box: The game has no map and gives you no hints and not much of a sense of direction. It's very easy to get lost in this game without a player's guide.
- Scrappy Mechanic: No matter how many energy tanks you collect, Samus will always start with exactly 30 health when loading a password. To avoid Fake Difficulty and getting killed within seconds of walking in a room with tougher enemies, players must give in to Fake Longevity by farming weaker enemies for health (which takes even longer after getting your first missile expansion, since enemies will often drop missiles instead of health regardless of how many missiles you're currently carrying).
- "Seinfeld" Is Unfunny: Playing the original game decades later can be rife with frustration, as Samus's jumping is floaty, you take lots of knockback from damage, you can't crouch or shoot diagonally, many of the rooms look the same, there's no map you can reference, and you can only save when you die, after which you must write down a complicated password and then grind back all the health you've lost. However, this was the first ever platformer of such a scale, back when going any other direction than right in a Platformer was unthinkable. While the environments don't change up much over the game, this allowed for a very expansive overworld for an NES cart, by far being one of the biggest games for the NES, and helped set the Metroidvania genre into motion. Thankfully, a remake is available that makes the map less repetitive and brings in the mechanics standardized by Super, though some fans argue that it loses the challenge and mysterious air the original has.
- Suspiciously Similar Song: The music playing in Ridley's Lair and Tourian sounds like the Castle music from Super Mario Bros..
- That One Boss: If Ridley and Mother Brain are too easy for you, then Kraid will more than make up for it. His attack pattern is merciless—he is constantly shooting spikes from both the front of his body and spikes above his body, with only a brief pause every three volleys of attacks—unless you have superman reflexes or patience, you will take a lot of damage during the fight. It only takes 25 missiles to kill him, but his stomach spikes can easily block your shots and missiles (and while you can freeze the belly spikes, doing it before he launches them can actually make him harder to fight), and he can quickly turn around if you get behind him, leaving little room for you to attack him. And if you die while fighting him, he can switch to a second attack pattern the next round, which can catch a new player off guard.
- That One Level: Lower Norfair / Ridley's Lair. It has some of the nastiest enemies in the game and is guaranteed to rip any newbie who dares set foot in it without the Varia Suit or Screw Attack into wet pulp. Thankfully, Ridley himself is ridiculously easy compared to his level.
YMMV / Metroid 1