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.
Base Breaker: Edward. Either one loathes him for his weakness in battle coupled with his angsty attitude or likes him for his spooniness and joining the characters despite being far from a warrior. The remakes have boosted Edward's popularity by making him and his abilities much more useful—to say nothing of the sequel.
Depraved Mad Scientist Doctor Lugae proves to be the template for monstrous scientists in the series, who transformed Edge's parents into chimerae, as well as gleefully altered his composition into a monster himself. His actions disgusted his superior, Noble Demon Rubicante, to the point where the latter had to explain to the party he was just as offended as them with Lugae. Remarkably, he has himself very little screen time, but he nonetheless makes very good use of it.
The true monster villain of the game is Zemus. He was the only voice pushing for world domination among the Lunarians, same bloodline as Cecil and Golbez/Theodor. Zemus was forced by his own people to fall into a millennium-long coma to allow humanity to evolve until the two could coexist peacfully together. Even in this state, he is still capable of manipulating Golbez to carry out his unsettlingly extreme dreams of an all-Lunarian utopia, which involves not only committing the atrocities throughout the entire game, but also using the artificialabomination Giant of Babil capable of dealing massive amounts of racially-based genocide on a global scale. Besides Golbez, Zemus also manipulates Kain during his fight with Cecil, who has turned into a paladin. It is also strongly implied that Zemus was responsible for the brainwashing of the four essences' incarnations, thus being responsible for the atrocities committed by the Elemental Fiends.
Ensemble Darkhorse: Kain. Highwind. Not only the most popular character in the game, but also one of the most famous and popular characters in the series, and a Memetic Badass to go with it.
Rydia also has a large fan-following. She and Kain evolved into Breakout Characters thanks to The After Years where they play much larger roles in the story than Cecil or Rosa.
Though his fanbase isn't as large as Kefka or Sephiroth, Golbez is pretty popular too. Being what is basically the Final Fantasy equivalent of Darth Vader will do that for you.
Game Breaker: In the GBA version, the enemies move at a much slower pace, and the battle system will occasionally give you extra turns. In addition, some of the items from the bonus dungeon (Kain's lance, for one, which randomly casts an unresistable version of Tornado and reduces a target's HP to single digits) are beyond overpowered. Anyone else want to defeat the Bonus Boss in two hits? Some of the battle system bugs were apparently fixed or at least toned down for the European release and/or later versions in general.
Goddamned Bats: Literal bats. They can come in groups up to six, are fast, all act at the same time on their turns, and use only one move; Bloodfeast. An attack that inflicts Sap, drains HP, and takes two seconds to complete the animation for. An omega-class annoyance, they are.
Also the Tiny Toads in the Sylph Cave that just cast Toad over and over again. The DS release makes them even worse by resetting the character's ATB bar when you get turned into or back from being a Toad, thus causing half your party on average to miss their turn as they transform. However, Draw Attacks can rectify that for you.
Good Bad Bugs: The original American release had one regarding the Sylph summon - if Rydia's hit points were full, the recovery portion of the spell would restore her magic points instead. This became a go-to attack spell, as it would heal allies, damage enemies, and restore her magic all in one shot. This was corrected in later releases - which surprised some people, as it was thought for a while that this was all by design, given that Sylph is a sidequest-reward summon involving traversing a difficult dungeon and quite a bit of running around on the related sidequest.
Additionally, using the Warp spell to get back into the Dwarf Kingdom's Crystal Room, and claim the Dark Crystal there, would let you skip the Sealed Cave entirely (though you'd probably have to make up for the missed Exp and loot elsewhere.) The "Crystal Room Warp Trick" was also removed from subsequent releases.
The item duplication trick was among the most infamous examples of all time.
It Was His Sled: Cecil becomes a Paladin. Heck, the remakes incorporate his transformation into their openings! To a lesser extent, Golbez is Cecil's brother. Dissidia: Final Fantasy makes it the central plot point of Cecil's story arc, and The After Years of course brings it up too. The DS version takes it further still by including Paladin Cecil in the character guide at the start of the manual, explicitly explaining that Cecil will overcome a trial to turn to the side of good,.
On the other side of the ocean, Japanese fans have clung to Golbez's "Iidesutomo!", said as he dual-casts Meteor with FuSoYa. It was a bonus voice for his EX Burst in Dissidia.
There's also some Fridge Brilliance - "Spoony" actually isn't a completely nonsensical word, it's just a rather archaic word describing being enamored with something silly, or being foolishly in love....and that actually fits how Tellah sees Edward!
To quote Word Of God "We checked and he really was Spoony" when questioned about why they put Spoony back in after taking it out.
Mis-blamed: While "You spoony bard!" and the other quirks of the original SNES translation are often attributed to Ted Woolsey, in reality he had nothing to do with the game's English translation. Square's policy at the time was that all localisations had to be done by either the original development team or someone at Square Japan's offices, as they felt that relying on foreign translators would hurt the integrity of the localisation. Ironically, though, the bad reception of this game's translation led to Woolsey being hired to localise the remainder of Square's SNES-era games.
And likewise, many things that were added to the script in later remakes (like the DS version) weren't new additions; they were actually just restoring cut-content. (The SNES version had roughly 25% of the script written for the game, due to text size constraints.)
Narm: On the one hand, the original Super NES release gave us "You Spoony Bard". On the other hand, when Tellah uses Meteor against Golbez, his reaction is the entirely out-of-character "No way!"
Never Live It Down: No, people who've only skimmed the wiki, Kain is not the poster boy for Heel-Face Revolving Door. Stop making the jokes. Even Square-Enix won't let him live it down — one of his cards in the TCG has the effect of him changing control between players when the opponent plays a Dark-elemental Character.
Rosa can't live down getting sick and kidnapped and is often accused of having no trait other than being in love with Cecil... even though she is just as vocal about the troubles in Baron and how to deal with the situation as the others when she's in the party, if one cares to pay attention to her actual dialogue.
Nightmare Fuel: Has anyone ever given thought about the notion of landing on a moving airship from a large height when that airship is driven by giant propellers?!
Older Than They Think: Many elements and "innovations" that are accredited as originating in Final Fantasy IV actually first appeared in the two earlier installments in the series, Final Fantasy II and Final Fantasy III, which were only officially released outside of Japan 13 and 15 years respectively after Final Fantasy IV on the SNES (as Final Fantasy II).
When Palom and Porom turn themselves to stone, Tellah says that it can't be undone due to being petrified by their own will (despite Porom, being a White Mage, can't actually cast Break). When the Giant of Babil awakens, they show up, perfectly fine and claiming that the Elder unpetrified them.
Oh Crap, seems like Cecil & party just got their asses kicked by Zemus. That means the world is DOOMED! Oh, wait, no... the secondary characters managed to revive them by prayer.
Rescued from the Scrappy Heap: Various remakes and The After Years have tried making Edward more useful, which has mitigated the hate somewhat. Being subject to Final Fantasy's most famous meme doesn't hurt either.
The Scrappy: Rosa. She suffers from Never Live It Down in regards to being a Damsel in Distress. She spent the second half of the game in the main party and is an invaluable healer, but for the second and third dungeons, she's sick, then after being cured goes through a single dungeon with the party before being kidnapped. There's also Overshadowed by Awesome—Rydia is generally a far more popular character and is the focus of several sidequests with her Eidolons, while Rosa's importance to the overarching plot of the game decreases greatly once she's rescued from the Tower of Zot. The After Years didn't help by making Rydia and the Eidolons central plot points while letting Rosa get kidnapped and saved again.
Seinfeld Is Unfunny: The game was a huge departure from the previous games at the time. The cast of playable characters is vast, their personalities developed and unique, it was the introduction of the Active Time Battle system, and the story is far deeper, the gameplay being driven by the plot instead of the player being free to wander the world. In short, this is when Final Fantasy truly began to develop its own profile as a game series. Of course, the series has since surpassed the original game in story, character and gameplay complexity, and the game is commonly criticized today for a simple and straightforward story, shallow characters and little gameplay flexibility.
That One Attack: Zeromus has an attack called Big Bang, dealing huge damage and causing continuous HP loss for any party member that manages to survive. It's the majority of his offense, but it's all he needs.
Bad Breath inflicts several disabling status effects on its victims at once. Getting hit by it can leave the party helpless.
However, in this game Bad Breath can be blocked entirely just by having protection against any one of the statuses it inflicts. Got a Ruby Ring (which only resists Pig)? The Malboros are sitting ducks. It wasn't until VIII that Malboros became "run on sight".
They Just Didn't Care: The Complete Collection has several cut corners in both the original game and The After Years.
Both games reuse sprites from the PSP releases of I and II. Not so bad in some cases, but the Alligator-type enemies used to be significantly larger than others, and with the usage of their II sprites they look out of proportion.
As covered under Bonus Feature Failure on the main page, the Interlude bonus story has less effort put into it than a fan hack, with the enemies being exactly the same as in the base game, encounter groups being mostly the same, and you can count on both hands how many new enemies were in the game. How much did they not care? Enemies drop equipment for characters that aren't even playable in the Interlude, like the Flame Lance for Kain.
Tier-Induced Scrappy: The characters that get shuffled out (Palom, Porom, Edward, Tellah, and Cid) are inferior in skill to those who become your final party members. Palom and Porom actually subvert this; together they're quite powerful, and Palom is actually superior at casting Black Magic to Rydia (having higher Intellect, an ability Bluff to boost it further and learning spells faster, though he lacks the better Summons). The only party member not affected by this is Yang, who can actually join in the final battle instead of Edge in the GBA version.
Tellah gets weaker every time he levels up, and when he rejoins your party, his MP pool is fixed at 90 points, which gets annoying since ethers are uncommon and he has access to high-level Black Magic spells (Firaga, Tornado, Bio, etc.) about a fourth of the game in (the Firaga/Thundaga/Blizzaga spells are 30 MP alone). His health is also pathetically low. It correlates with his old age, but still.
Cid's only real attribute is his decent strength, which is still lower than Yang's or Kain's. His Study skill (which tells the HP of enemies) isn't that great either, and doesn't work on many bosses.
Edward can actually be an aversion of this. At high levels, even with lower HP than Rydia, he is one of the fastest members of the party and his harps deal full damage from the back row. His weapon received from the Bonus Dungeon deals extra damage to enemies weak against fire and dragon-type enemies, of which there are several in the final dungeon—how does 9999 damage from the Spoony Bard sound?
Strangely, Cecil himself becomes this not long after he becomes a paladin. He has well-rounded strength, which is lower than Kain's and can get overshadowed by Edge's ninjutsu abilities, and he has access to basic White Magic, which is useless when Rosa, the game's actual White Mage, is around. However, he does remain the most durable member of the final five, with arguably the best equipment options.
What an Idiot: Cecil borrows a world crystal (which are pretty damn important, and which are being gathered for unknown nefarious reasons) to give to Golbez in exchange for Rosa. He doesn't try to create a fake or trick Golbez or even make sure Golbez gives Rosa back first; he just hands it over. Guess whathappens next. And hell... the entire party in the Tower of Zot, especially when you read Yang's thoughts in the DS version—he rightly thinks that Kain's offered deal is fishy as hell and doesn't say a damn word.
Woolseyism: The DS version, courtesy of Tom Slattery. The Eidolons were previously just "summons" but he wanted them to have a proper name like summons did in other titles, so he dug up "Eidolon" from Final Fantasy IX, and he also translated the Mysidian Legend to more clearly refer to the duality of Cecil and Golbez. This is also a case of Doing It for the Art — Slattery was a fan of the game and the original plan was to just dump the translated text from the game's GBA release, but Slattery offered to redo the translation from scratch in order to be more faithful to the Japanese and give it more flair.