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.
Alternate 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.
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 Brady Game's strategy guide for Kingdom Hearts II, the reason why it was included in the subsequent games was because of it's overwhelming fanbase. It even got some the merchandise based on that single world.
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 will undoubtedly soldify 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 off-set 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.
Goddamned Bats: Many of the Heartless fought in the 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 unrepentantly 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.
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).
Inferred Holocaust: 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.
Most Annoying Sound: Queen of Hearts? More like Queen of Needstoshutthefuckup. GET THEM, YOU FOOLS! GET THEM, YOU FOOLS! GET THEM, YOU FOOLS! GET THEM, YOU FOOLS!
On the plus side, you can vent your frustrations against the queen by locking onto her fat ass and whaling on her, which knocks her out for a minute or two.
Also, Pinocchio when he's trapped in Parasite Cage. GET ME OUTTA HERE!!! HEEEELP!
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 KH2, since Cure/Cura/Curaga drains all of the user's MP and gives them a 60 second recharge time.
Nightmare Fuel: Most of the stuff in End of the World, and several things in Hollow Bastion.
The destruction of Destiny Islands.
The only way to land in Monstro is to let the giant angry monster whale swallow you whole.
Not to mention that when it happens, you first see the Space Whale "swimming" on its own, then it spots you and swims towards you, and finally opens its mouth. It gets worse when you see everything from behind the Gummi ship, and therefore you get to see Monstro's mouth as it covers the entire screen. Combine that with the final Fade to Black and what you get... isn't a pretty image.
You can travel from Monstro's bowels to his mouth in one passage. Cheers for that image, Square Enix.