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.
There's also Kain/Rosa for those feeling more sympathetic towards Kain, at least.
Rydia/Kain is also a fairly popular pairing.
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.
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?!
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 overarcihng 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.
Interestingly, when Rosa gets the Holy spell, she can do more damage than most of Rydia's Black Magic spells, and even a bunch of her summons, including Bahamut. Then again, it's her only offensive spell.
Also Edward. In battle, he has poor stats, flees when in danger and his abilities are highly situational at best. Add in the fact that he's fairly angsty (though with good reason) and you'll find nobody was sad when he left the party.
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 rather blatantly reuses 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, but with the usage of their II sprites their sprite is the same size and thus they look out-of-proportion.
And as covered under Bonus Feature Failure, 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? The enemy groups may drop equipment that your party can't use, like the Flame Lance for Kain.
Tier Induced Scrappy: The characters that get shuffled out (Palom, Porom, Edward, Tellah, and Cid) are less superior in skill than those who would 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 40 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.
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.
Values Dissonance: "Gawd, Edward, how dare you cry over the deaths of everyone you've ever loved? Can't you be like the rest of the incredibly stoic and manly and not-at-all-wangsty leading cast? You such a bitch."
Wangst: Cecil and Edward, Up to Eleven if you care to read their thoughts in the DS version.
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.