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.
Big Lipped Alligator Moment: The "Overworld" room in one of the bonus dungeons in the remake. It's the Overworld... except with treasure chests, NPCs, stairs to the next floor, and a lot of other weirdness. You can even find another airship there.
Demonic Spiders: Sorcerers and anyone with paralyzing powers, such as Ghasts (renamed Geists) and Ghouls. To a lesser extent, anything that could turn you into stone as well, such as Cockatrices and Medusas. The chance of working was rare, but the curative for the condition was both expensive and couldn't be used in battle.
The remakes with additional content add more enemies that can kill in one hit, and make Sorcerers (renamed Mindflayers) able to use the instant kill attack on the entire party at once.
The Mages (Dark Wizards), which only appear in the Ice Cave and have the power to cast Rub (Death) on any party member. They also repeatedly cast Fir3 (Firaga), Lit3 (Thundaga), Bane (Scourge), Slo2 (Slowra), and Stun. Good luck surviving these guys.
Many enemies with the ability to poison characters that are extremely slow to defeat with physical attacks, namely the Scums (Green Slimes) and Slimes (Black Flans), which have the highest defense stats of all characters in the game, have a very rare chance of being defeated with physical attacks in one hit, and whose only weakness is Fire spells.
The Sentry (Soldier), seen only in the Sky Castle (Flying Fortress), which is tough to defeat physically with high defense stats, but its attack stats are even worse. They are able to defeat any party member for at least 100 (and sometimes as much as 200) HP in a single physical attack and are always paired up with their lesser cousins, Guards (Guardians).
And then there are the MANCATs (later Rakshasas), each of which had a high chance of casting FIR2 (Fira). Being surprised by 8 of these could result in a Total Party Kill.
Mummies and WzMummys (renamed King Mummies), which can put your party members to sleep.
Ghosts, which appear in the Sunken Shrine, appear in groups of as many as 5, and can damage any member for 100 HP on average. Like Ghasts and Ghouls, Ghosts can paralyze you. One of the few recurring enemies that are impossible to flee from.
Wizards (Piscodemons), another of the few recurring enemies you can't flee from in the game. First appearing in the Marsh Cave, Wizards will damage your party members on a one by one basis for more than 50 HP.
The Cockatrices' cousin, the Pyrolisks, are usually found in the Gurgu Volcano (Mt. Gulg) and possess the Squint spell that instantly kills a party member. Cockatrices and Pyrolisks are sometimes paired up together. Fortunately, Cockatrices are weak to Fire spells and Pyrolisks are weak to Ice spells and can work, assuming none of your party members are killed.
The Sorcerers (Mindflayers) hit for only 1 HP damage, but with the added effect of a potential instant KO.
Evil Is Sexy: Marilith/Kary. She's also Creepy Sexy, since she's a giant snake woman.
Game Breaker: The Monk/Master class, especially if you stay two or three levels above the expected level. It's very possible that the nunchuks were added to the game in an attempt to stop players from realizing how unbalanced the class was. The Monk is always more powerful when using his hands, since he's the only character in the game who dual-wields.
It should also be noted that this is only when a Monk/Master is part of a balanced team. Without a magic user or two in the party, you're screwed, especially if you're going for an all-Monk run.
The Monk gets fairly ridiculous in the GBA remake. A Fighter won't out-damage the monk (or even come close to its damage output) until he gets the Infinity+1 Sword, and even with said equipment, it's not by much. Plus, all the armor Monks need are a ribbon since, thanks to how they work, going naked is equivalent of having the evasion of a ninja, and they still have decent defense. Magic defense and status effects are pretty much their only (and biggest) concern, both of which are covered by the ribbon.
From the remakes (this was bugged in the original NES version), the Temper (TMPR) spell, which increases the physical attack strength of the targeted character. There is also the Haste spell, which doubles the targeted character's speed, and the Giant's Gloves, which casts the Saber spell at no MP cost. That Temper and Haste each have a lower MP cost than Flare in the GBA version doesn't hurt, either.
In the GBA and PSP remakes, there is the Judgment Staff, which casts Flare at no cost when used as an item. The GBA version also gives you a chance to obtain multiple copies of it.
The Barbarian's Sword from the Anniversary Edition is the strongest weapon in the game by a country mile. It's also equippable by the Red Mage. If you manage to snag two of them with a Fighter + Red Mage (or party with two of either)... let's just say that Warmechs become easy pickings.
The Red Mage doesn't have any real limits on his magic compared to White and Black Mages, meaning there's no reason to actually use the latter two.
Goddamned Bats: The Hellfire Chasm dungeon in the Dawn of Souls and Anniversary Edition remakes has a floor where multitudes of bats can block your way from finding the exit to the next floor. Did I mention that said floor is primarily composed of tiles that will continuously sap the party's HP as long as you're not in the few safe zones in that floor?
Good Bad Bugs: It's possible to land the airship directly on top of the caravan instead of fighting across the desert. The caravan was the only "unlandable" spot that could actually be landed on with the airship. However, we can't be sure this "bug" was intentional or not.
HEL2 (a 5th-level spell), has the exact same curative power as HEL3 (a 7th-level spell) when used in battle, and when you consider that the cheapest way to heal out of battle is potions, you're better off skipping the latter and saving yourself the 45,000 gil.
The "Peninsula of Power": A four square peninsula tip that is the closet point in the southern continent to the northern one. Due to a mapping mistake, those four squares had the enemy encounters of the continent to the north, allowing you access to much higher-level— but still kill-able if fought with full HP/MP— monsters long before you should be able to face them, allowing for some serious Level Grinding. Yet another bug that has not only been kept in virtually every remake but also gave us the Peninsula of Power Leveling trope.
Guide Dang It: In the original version, several important pieces of info were left out of the manual. For example, there are several items that can cast spells in battle (one of the most important - the White Shirt could cast INV2, a sixth-level spell that greatly enhances your party's evade, for free), though the game and the manual give absolutely no hint as to what does what. Some could be guessed (the Thor Hammer casting LIT2, for example), but most required trial and error.
Hilarious in Hindsight: Those complaining about the effeminate faces of the more recent protagonists would do well to remember that White Mage's gender was also ambiguous in the original. And even in the remake, if you make him male, he looks like a Bishounen.
It's Easy, so It Sucks: One of the biggest complaints about the GBA and PSP remakes. To clarify, the magic system was replaced with the traditional magic points, experience is gained faster, Phoenix Downs were added, saving is possible almost anywhere, and a characters' attacks now redirect to another enemy if the original target is killed by someone else the same turn.
In fairness, a portable RPG practically mandates the ability to save easily, since one may need to stop the game at any time.
That One Attack: Two— Astos's Rub (Death) will kill one party member instantly, and WarMECH's Nuclear attack deals huge damage.
Rub actually only has a 75% chance of hitting. It is entirely possible to get through the fight without a single party member dying. It is also entirely possible for Astos to rub out three of your party members before you can beat him if you aren't sufficiently leveled. It really becomes that one attack after you get past the area, because you can get equipment that makes you immune to rub after this fight.
And the Sorcerers'/Mindflayers' regular physical attack has a high chance of insta-KOing whomever it hits.
That One Boss: Astos. He's got RUB (Death) and he ain't afraid to use it. A lot.
That One Level: The Marsh Cave, which dictates that you stock up on Antidotes to cure poison status caused by the many poisonous enemies in this dungeon, not to mention the area surrounding it on what is a long walk from the nearest town (Elfheim), and on Potions for a dungeon that is not only literally full of Goddamned Bats, but also the SLIMEs (Green Slimes) that require you to have Fire spells on standby, undead enemies that repeatedly paralyze you, and the WIZARDs (Piscodemons) that require you to have Thunder spells on standby. Good luck getting through the Marsh Cave.
The Terra Cave, with its Cockatrices and their petrification abilities that dictate stocking up on SOFT Potions (Gold Needles) (sold at a high cost and only in one town, Elfheim) to reverse the affliction. There are several Goddamned Bats that aren't enemies in this dungeon (or any other), but get on your nerves when they block its narrow passageways.
The Ice Cavern, which gives you lots of Demonic Spiders that often attack in groups of at least 3: the return of Piscodemons and Cockatrices, the infamous SORCERORs (Mindflayers) and their one-hit instant KO attack, the MAGEs (Dark Wizards) who appear in groups and spam FIR2 (Fira) and RUB (Death) on your party, and some more undead enemies out to paralyze you. This is a level that's not even significant to the plot and which you're only doing just to obtain the Floater (Levistone) that will grant you access to the airship. Oddly enough, it applies when many gamers use this in an example of Sequence Breaking by completing the Ice Cavern (and the Citadel of Trials) to be able to get the class change before entering Mt. Gulg.
The top floor of the Flying Fortress, which has the WarMECH (Death Machine) and more appearances of the Mindflayers that now come in groups of up to 7, on the very same floor you face off against Tiamat.
Viewer Gender Confusion: White Mage is actually intended to be male; this is noticable after being upgraded to the White Wizard. However, you could argue all of the characters are androgynous enough to be whatever gender you want.
Except for Monk in the Dawn of Souls remake; he's Always Male.
In the Spanish version of Final Fantasy: Dawn of Souls, the White Mage is refered to as "Maga Blanca". "Maga" is a gendered noun that refers only the a female mage; and "blanca" is the female version of blanco, wich means white, so at leastshe is female in Spain. If foreign translations and localizations should be considered canon, that's another subject.
The Red Mage is almost as ambiguous as the White Mage.