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.
Alternative Character Interpretation: In the beginning section at Destiny Islands, Riku and Sora's competitive relationship comes across differently depending on whether the player is superior, equal, or inferior to Riku.
Base Breaker: Riku was this when the game came out, for being a complete dick to Sora and somewhat naively turning against his former friend. With the release of each subsequent game, this went away due to Riku improving his attitude.
Best Level Ever: There just hasn't been a world in this series yet that has been able to top this game's visit to Hollow Bastion in awesomeness.
Aside from perhaps the sheer unnerving beauty (and frequent terror) of End of the World, the game's final world.
Canon Discontinuity: The estate of Edgar Rice Burroughs won't allow Deep Jungle to be used or even referred to in subsequent games. As a result, the events of the world have been retconed and the series acts like the world never existed. Not that fans really cared for it.
Draco in Leather Pants: Riku while he's on the dark side. His fangirls just can't seem to admit he was ever in the wrong, and usually blame shifts to Sora or Kairi for some absurd reason or other.
It's worth noting that his Dark Mode clothes even look like they are made of black leather or muscle tissue...
Why Chernobog was included. This also applies to his reception in the series- he's regarded as one of the candidates for the series's example Best Boss Ever, at least thematically. When a Fantasia world was announced in Kingdom Hearts 3D [Dream Drop Distance], people had hoped he'd make an appearance. They were not disappointed.
Out of the additional party members, Beast and Peter Pan seem to be the most popular, largely due to the fact that they're the only two who are actually helpful in battle.
Halloween Town also developed a strong fandom. In fact, according to BradyGame's strategy guide for Kingdom Hearts II, the reason why it was included in the subsequent games was because of its overwhelming fanbase. It even got some the merchandise based on that single world. Ironically, it was the only worldnote excluding the now-illegitimate Deep Jungle to not appear in the manga.
Epileptic Trees: The Another Side, Another Story secret ending from the Final Mix version. Fans analyzed this video to death, getting crazy theories out of practically every single frame and setting internet forums on fire. The new Ansem Reports were a big source of Fan Wank as well. Although all theories were Jossed, fans never stopped to come up with theories about where the series would go in the next installment.
First Installment Wins: The series goes on and quality is always debated, but the debut installment seems to be destined to be the one everyone will remember the most due to the sheer hype and reaction surrounding the concept.
Also, to this day it has sold the most out of the whole series in North America, Europe, and Japan (counting the Final Mix edition), and undoubtedly solidified this status with 1.5 ReMIX.
Beginning the game with defensive priority unbalances the game in the following ways: As you level up, you gain additional hit points, you have early access to an ability that nullifies death-dealing critical hits, you have early access to a Dodge Roll that makes you extremely mobile, and on top of all that, your Cure spell can easily offset any damage taken in battle, plus you can easily regenerate MP based upon your damage taken. In short, you are a Lightning Bruiser with unlimited healing on top of being difficult to kill. Even "Those Bosses" required to beat the game can not hope to out-damage a competent player fast enough.
This is in addition to the fact that you get Lucky Strike earlier than other setups, letting you gather the ingredients for the Ultima Weapon quite early in the game.
No mention of Bambi? Bambi is one of the greatest summons in Kingdom Hearts due to his ability to give you a RIDICULOUS amount of MP orbs which basically means you can spam magic for free while he's there. Did you run out of magic? Just take a second or two to pick more MP orbs and you're full again. Not only that but, depending on which world you are, the more enemies you beat while he's there and the more items you'll be rewarded including various Potions, Ethers and materials. He's also available once per room meaning you can see him quite often.
Tinker Bell also merits discussion. Unlike the other Summons, Tink does not replace any allies on the battlefield and lacks a depleting MP bar, meaning that she can only be dismissed manually. Tinker Bell will constantly heal Sora and his allies while active and will revive Sora, completely restoring his HP, should he fall in battle before departing. Combined with Second Wind and other Support Abilities, there's little you'll need to worry about as long as you're sufficiently leveled, and Tink's Regen effect largely counteracts the need to spend MP on healing, meaning Sora is free to use Aeroga, Graviga, Strike Raid, Ars Arcanum, etc. instead. Of course, you can't perform any Summons in fights where Sora is down at least one partner and Neverland is the third-to-last world traveled to, but she comes in handy for tough lategame battles like Maleficent's dragon form and Chernabog.
Many of the Heartless fought at End of the World are either this or Demonic Spiders, but the Angel Stars take the cake. They fly high above easy reach, take far too many Keyblade hits to die, and have an oh-so-charming tendency to block all attacks whenever they feel like it. Not terribly dangerous, but annoying as all heck to get rid of.
In Final Mix, the Sniperwild, a purple version of Bouncywild, utilize lock-on crosshairs, and if they connect, can instantly hit you with a bullet from anywhere. And they DON'T STOP SPAWNING. So your best bet if you're running out of MP for Cure spells is to run to safety.
More like Demonic Spider with that one. Considering they will spawn relentlessly to kill you if they see you meaning the only way to get their synthesis drop item is to rely on the short-lasting Stop series of spells. Good luck.
Fat Bandits, if you're a lower level, and especially in the Coliseum tournaments. The Stop spells help, but only so much.
Black Fungi, hostile versions of the White Mushroom, emit poisonous clouds which continuously drain your health, and they can turn to stone and make themselves invincible for a period of time.
Ansem: Look at this tiny place. To the heart seeking freedom, this island is a prison, surrounded by water. And so, this boy sought to escape his prison, and he opened his heart to darkness.
As noted on the Dramatic Irony page, Ansem's manipulation of Riku becomes even worse when you remember that Terra not only made Riku his successor, but promises to someday show him the worlds that exist outside of the Destiny Islands.
In Atlantica, King Triton exiles Sora because he claims that "the key bearer shatters peace and brings ruin". He must've heard about the Keyblade War.
Anyone who's seen James and the Giant Peach would recall the undead pirate scene with Jack Skellington and Donald Duck. Well, they can both fight alongside each other in this game while in Halloween Town (though Donald is a Mummy instead).
Even more funnier in France, since Sora and James were both voiced by Donald Reignoux in the French versions of their respective games/films.
The Heartless invasion. Although this fact is pretty heavily glossed over and not really discussed much, it is stated at least once that it has completely destroyed several entire worlds by the time the game starts, presumably killing hundreds of billions of people. The few survivors in Traverse Town are, for all intents and purposes, interdimensional refugees, condemned to spend the rest of their existence well and truly homeless, huddling together and trying to preserve what remains of their culture and civilization. It's basically an After the End scenario on a hilariously massive scale.
Birth by Sleep Final Mix reveals that worlds consumed by darkness aren't necessarily destroyed... but are now stuck in a bleak, hellish dimension. A Fate Worse than Death, perhaps?
All of the lost worlds are restored via a Reset Button, pushed by none other than Sora; additionally, the Summon Gems are stated to contain spirits that survived the destruction of their world. Two of them appear in Kingdom Hearts II.
Magnificent Bastard: Both Maleficent and Ansem. Ansem happens to be the better player, though both of them end up being pretty damn Genre Blind when it comes to the nature of light and darkness.
This game predated auto-restock, so hearing your other two party members call out Sora's name at the same time in battle can get pretty annoying because it means they both just used an item on him, likely the same one. And Sora can heal himself.
A variant of this is also very annoying: hearing someone call Sora's name just before or immediately after you heal yourself, because you probably just used an item (or magic, which is recoverable) for nothing. Made even worse in KHII, since Cure/Cura/Curaga drains all of the user's MP and gives them a 60 second recharge time.
Paranoia Fuel: Okay, who else got ambushed by Heartless at the Rabbit Hole in Wonderland? You may suspect it when you notice that the save point is the same temporary version in the Rose Garden where you fight the Queen of Hearts...
But the Heartless attack on Destiny Islands is just cruel (at least in the PS2 version). If you dillydally too long in the beachside shack? THE SAVE POINT DISAPPEARS AND THE HEARTLESS ATTACK!!
When IGN did their top 100 PS2 games of all time list, Kingdom Hearts broke the top 10 and came in at #7.
That One Achievement: The Gummi Ship Collector trophy in the 1.5 version, which involves getting every Gummi Ship blueprint in the game. For the majority of obtaining the blueprints, this means fulfilling some truly insane mission requirements traveling between worlds with the Gummi Ship, like doing Atlantica's third mission (do not collect items, and get a high score of 260 points or more), in a minigame widely considered a Scrappy Mechanic prior to its revamp in Kingdom Hearts II.
That One Boss: Reports differ widely, but both Ursula battles, Maleficent-Dragon, and Ansem-Riku are usually brought up.