Re:Coded, while answering several questions about the KH universe, is oddly difficult. But, it's worth a play through.
Re:Coded starts out in Disney Castle. Jiminy comments that he has two journals, one of they're first adventure and one of they're second (referring to KH1/CoM and KH2). However, in the first journal, everything is erased except for a single line, "Thank Namine." However, looking through again, Jiminy finds a second line. "Their hurting will be mended when you return to end it." Confused, Jiminy brings this before Mickey, who's solution was to digitize the journal and find the meaning through a computer system. They create a Data Sora to help search through the journal and destroy Blox, data bugs that are messing with the system.
The best way to describe this game is a rehash of KH1. Data Sora travels through Destiny Island, Traverse Town, Wonderland, Olympus, Agrabah, Hollow Bastion, and Castle Oblivion. In that order. You are unable to choose the order, but you can revisit the different worlds. The worlds take they're KH1 appearances, and almost all of the Heartless you face are from KH1. The new addition is of the Dataspace. This world is very reminiscent of Tron. It's blue and digital looking and covered in Blox. In these multi-floored sections, you must use the Blox to solve puzzles and defeat bugged Heartless, who are digital looking versions of normal Heartless. You can also finish extra "missions" in each floor to gain more SP, which is used to buy items at the end of the section. I personally really liked these sections more then the rest of the levels. Though, that's just me.
Combat has been taken from Birth By Sleep with the return of Deck Commands. Basically, you swing the keyblade with A and then you can use items, magic, and skills set to your deck commands with X. You can cycle through your deck commands by pressing L and by holding L, you can manual move through the deck (though, I never used that feature). Camera can be controlled with R. By taping R, the camera snaps behind Sora. If you hold down R and press the buttons on the D-Pad, you can manual move the camera. This is a big problem during combat, as you can't move the camera and move at the same time. The Y button controls Blocking and Dodge Rolling. B is to jump. This control scheme is kinda annoying to get use to at first, but by the end of the game, you should be a pro.
As with 358/2 Days, the level up system is unique. You use a Stat Matrix, which is a big kind of...puzzle board. You have abilities laid around the board and pathways to each. As you level up and explore the Dataspace, you get little pieces that attach to the board, lighting up the pathways. When a pathway reaches an ability, it activates, giving it to you. Classic abilities like Scan, Combo Plus, and Glide are obtained this way. Also, you can unlock more accessory slots and deck commands this way. You can also unlock Cheats, which allow you to gain certain abilities and advantages, while making the game harder. You can also change the difficulty of the game whenever you want at this menu. It's a fun little system, though I still enjoy the Panel system of Days.
My dislikes. The game has really random changes in difficulty. You'd go through an area that is really easy and as soon as you walk through the door, you face stupid hard enemies. You also don't get a Cure spell till late in the game. The Camera work is terrible, as well. My biggest complaint is the first half of Hollow Bastion. Think, KH1 beginning of Hollow Bastion, but worse. Much worse. Also, Sora's vocal work is kinda terrible. As neat as it is to have the bosses in each world function differently, some of them work so that it doesn't matter what level you are, or the skills you have. All that matters is that you've played side-scrollers or shooters a lot. It's quite annoying.
Ugh, I have a love hate relationship with this game. For those who want to know what is in the bottle at the end of KH2, it's worth it. Also, the secret ending explains what will be going on in Kingdom Hearts 3D, the next game in the series. While combat is solid, difficulties raise and lower at random and redoing KH1 again can be a little tiring. The Stat Matrix is fun, as is the Deck Command system. But, this is probably a game you should borrow or rent first. Mix reactions will be had all around. Though, I enjoyed it enough.