The gameplay gets progressively more stylish and complex as the series goes by. First off, Kingdom Hearts and Kingdom Hearts II are basically button mashers especially on difficulties below Proud and Critical, though KHII added "reaction commands" and that's basically a more friendly version of QTEs. Chain of Memories and Re:Chain of Memories - basically a remake for the PS2 with a slightly better combat system - involve card combat. Sounds weird, but it's basically like playing Yu-Gi-Oh, Chess, or more strategy orientated games where you try to overpower the enemy's card(s) through normal attacks, special commands - adding certain cards together -, and duels. It's really weird, but that's pretty much when Kingdom Hearts becomes a more complex franchise, aside from the convoluted plot, theme, lore, setting, etc. Ironically, Chain of Memories was released before Kingdom Hearts II; Square Enix went backwards, essentially.
Next up are coded (and its remake for the DS: re:coded), 358/2 Days, Birth By Sleep, and 3D Dream Drop Distance. coded was for mobile phones and apparently it follows the combat system of Chains of Memories, I don't really know since it sort of goes unnoticed even though it adds to plot detail. Similarly, 358/2 Days just covers what happens between Kingdom Hearts, Kingdom Hearts: Chain of Memories, and Kingdom Hearts II. Gameplay-wise, I don't know much about it.
Birth By Sleep, however, is when the series ramps up the difficulty by a lot. In the past, most of combat was pressing one button and/or shifting to magic for healing, support, etc., along with the occasional summon and Drive - a temporary transformation, sort of like going Super Saiyan, but with added weaknesses, strengths, and increasingly shorter time in more powerful forms. Otherwise, with Chains of Memories, you combined cards and played "chess" to fight. Birth By Sleep takes that all a way. Can you button mash? Yes, maybe on Easy mode and the beginning of Normal mode, but on Proud and Critical, you're probably better off running instead of button mashing. Anyway, you have a dedicated attack, jump, guard, and dodge buttons. The other button, I think it's triangle, is dedicated to special moves ranging from Fire, Cure, Mines, and special combos. Along with that, you have "Command Styles" and "Dimension Links" which basically either adds elements to your attacks or changes how the character functions for the time being - like switching to critical hits only, getting a giant sword, phantom swords, etc. Also, there's Shotlock, which is basically like using Ultimate Techniques in Ninja Gaiden or certain moves with I-Frames to deal a lot of damage without getting hit, but if you screw up or when it ends, you're probably going to be vulnerable to getting combo'd and dying. That's Birth By Sleep in a nutshell.
3D Dream Drop Distance basically is a toned down version of Birth By Sleep. I don't know if there were complaints about the difficulty of Birth By Sleep, which I don't understand since if you're playing on Hard, you should know what you're getting yourself into. The other reason is through plot devices: in this game, you're basically playing as characters who are training to become Keyblade Masters and characters who have already proven themselves to be capable in combat. Also, these guys are more well-balanced compared to the characters in Birth By Sleep; you had a slow, heavy-hitter and basically Dark Knight from Final Fantasy - people hated his dodge, it's basically Dante's dash in DMC3 except sort of slower and with less I-Frames compared to the other two -, a speedy, though weak character with some incredible area of effect attacks, and a powerful mage who is good, but not amazing fighter - also, she has a "broken" dodge that has more I-Frames compared to the others. And in this game, you had party member who could function as distraction while giving some "party attacks", whereas in Birth By Sleep, you were alone for pretty much 85% to 95% of the game. As for new mechanics, 3D Dream Drop Distance adds a "catch 'em all" monster party system and a pseudo free-running system called "Flowmotion"; it just launches your character from wall to wall or lets you interact with poles and rails along with Flowmotion-only moves. It basically replaces Command Style, Dimension Link, and Shotlock.
Story-wise in terms of games since there are "movies" which connects some games together, it goes from Birth By Sleep, Kingdom Hearts I, 358/2 Days begins, Chain of Memories, 358/2 ends, Kingdom Hearts II, coded, 3D Dream Drop Distance, and Kingdom Hearts III. So, if you want to play it chronologically, try this. If not, then just go through the release orders of the games. I do recommend that you try Kingdom Hearts on the PS2 first and see how you like it since there are two HD re-releases of the games and they cover everything but 3D Dream Drop Distance which I think has a contract with Nintendo - kind of make sense since it was released like a year ago.
For the HD re-releases, the first is out, it's called: Kingdom Hearts HD 1.5 ReMix and that contains Kingdom Hearts I Final Mix, Re:Chain of Memories, and 358/2 Days as a movie so you don't have to "suffer" through it. The second will be out sometime in 2014 and it's called Kingdom Hearts HD 2.5 ReMix and it contains Kingdom Hearts II Final Mix, Birth By Sleep Final Mix, and re:coded as a movie. Final Mix, by the way is basically an International Edition like with some Final Fantasies, and it just adds new weapons, enemies, bosses, abilities, and plot scenarios. I really recommend getting the HD versions of the game because Final Mix basically changes the games from being cool to awesome. Also, since 358/Days and re:coded are DS games and that Birth By Sleep only comes in a physical format for the PSP, it's probably going to be easier to just get a HD re-release than track down those copies of the game along with a PSP since PS Vitas only play PSP games if they're digital, I believe.
Speaking of Kingdom Hearts III, so far from what we know, and considering that Final Fantasy XV, then known as Final Fantasy Versus XIII, was a "prototype" to Kingdom Hearts III, it probably will have a more robust combat system. When I mean prototype, I mean that anything that didn't fit in with XV could be transferred to the next Kingdom Hearts. Also, both seem to have similar combat systems except that XV seems to have stealth and shooter mechanics and that XV is much more violent and brutal compared to Kingdom Hearts. So there's that.
Actual story, well it's a series about light and darkness, the strength of ones heart and what strength really means. Also, friendship. It is cheesy, but lots of games from back then were cheesy compared to today's grumpy, hardcore, and edgy games. Thing is, this series starts off being biased about light and darkness - light is totally good and darkness is totally evil -, but as the series progresses, you learn that isn't true. Light is inherently a warm element in life and stories, but with too much light, it can blind you. Think of the people who thought they were doing good, but ended up doing much worse. Anarchists are tired of governments being corrupt and such, but they way the look at government are an extreme. Communism in its early stages want to raise the floor for the poor, but well, it didn't go so well when power corrupted the leaders.
With darkness, it's a mysterious, cold, and frightening element. People are generally afraid of what they cannot see. Also, darkness allows people to use more power with less restraints, but that comes at the cost of killing yourself from abusing it or hurting others. It's like the element of fire. In Avatar: The Last Airbender, fire is a simple element capable of swift destruction to anyone. Aang, being young and reckless, ends up burning Katara since he wasn't in control. The path of learning the elements for Avatars supposedly ends with fire or begins with fire since if you master the other elements, you will have learned the restraints and techniques that make up firebending or mastering firebending at first already gives you everything to learn the other elements. Darkness in Kingdom Hearts is kind of like that. Two protagonists have the ability to use darkness. The first, Terra, was tricked into believing he had darkness in his heart - technically, everyone does, but that's something for later - and Terra goes on a path to control his darkness. Unlike the other, Terra makes an effort to use darkness to protect others, but Terra is gullible - suspiciously gullible - and was lead down a path of corruption that ended with him being split into several beings with at least one being him and not just puppets. These events also leads to one of his dear friends believing that darkness is just hate, anger, and corruption. It also leads causes him to lose many people dear to him.
Riku is the other protagonist who uses darkness. He gains the ability after being seduced into thinking he needs more power. Riku ends up being concerned about power and that he needs power to save a friend. This ends up with him being possessed, depressed, and confused about who he is and what he is. Since he had help from a well-known mouse, Riku ends up learning to accept the darkness within him along with the light within him, though Riku does remain moody from time to time from his past and guilt. Also, Riku ends up inspiring said mouse that darkness and light are two interdependent things sort of like Yin and Yang. Both Terra and Riku are kind of like Kyle Katarn and Galen Marek aka "Starkiller" from the Star War series. These guys are all in touch with the "dark side", but are able to use it to protect others rather than harming others and for power.
Anyway, that's the central theme of it, but there's more to the story than that.
The appeal for me I think is Disney and Square Enix ideas mixing together. I played the first game when I was young and I still watched Disney stuff - nowadays, I more of a Looney Tunes and Tom & Jerry, guy, but hey. So, it's nostalgia for gamers to see Donald Duck, Goofy, Chip & Dale, Stich, Hercules, Hades, and Pluto alongside Cloud Strife, Auron, Zack Fair, Sephiroth - a superboss for two games -, and Yuffie. After that, it might be because it's a friendly game that lets you enjoy it without having to learn complex moves and such so you can just see how the story unfolds, but as the series went on, it probably turned into the opposite since the series did become harder. Also, the flashy moves are pretty much one of the closest thing we had to Final Fantasy VII: Advent Children back in the day. Unfortunately, with the development time and with most of series being released on handhelds, none of which I had ever owned, I could only catch up by spoiling the series and never being able to play them, until now with the HD re-releases, though I'm mostly in it for Birth By Sleep.
Also, one fair warning: remember this is a series that started in 2002, so the AI sucks. Hell, current AI still sucks in many recent games. They do get better, by better I mean they don't die as much and are basically making snail crawls to better AI. With next-gen coming up, Square Enix promised that Kingdom Hearts III and by extension, Final Fantasy XV, will have much better AI. Come to think of it, Final Fantasy XIII and XIII-2 had decent AI that could prioritize everything without player input. Then again, it's basically the gambit system without the player involvement. Still pretty cool though.
