Since I am going into this game "blind" I thought that I would offer my perspective. Note: I am not playing a preview copy and have not been compensated for my thoughts.
I'll admit that I'm a sucker for Disney and the original Epic Mickey was such a fun game that I actually completed the game twice. As established in the original game, Epic Mickey 2 carries over the paint vs. thinner "fighting" technique, and the outcomes of using one over the other affect the overall ending of the game.
The controls for Mickey are virtually identical to the original game, only this time you have a few added options when interacting with the second player, Oswald. Oswald generally stays near Mickey and follows him from place-to-place and will operate independently if only one player is active. However, at any time, Player Two can link up the Wii remote to take over control of Oswald. I haven't done this yet, so I can't comment (although other reviews lead me to believe that it utilizes a split-screen perspective, which is part of the reason why I'm not in a hurry to try it out). However, when left to be managed by the CPU, Oswald often lags behind when walking (which does NOT affect the ability to move forward, thankfully -- if he lags too far behind, he will reappear next to you in a poof of smoke) and is generally pretty useless in battle. The Oswald character has some really great moves (he utilizes a remote control, capable of shock) but Mickey can't command him to use them. The result is that he does and doesn't and often uses them in a counter-productive way. It would be much better if there was an option to assign Oswald to a task or even a spot in the room, even if it just meant keeping him out of the way.
The enemies in this game also seem a little uneven. Nearly from the beginning, we encounter more challenging enemies that were seen in the later levels of the original Epic Mickey and then they sort of level off to the relatively "easy" enemies. It left me with the impression that you could not easily pick up Epic Mickey 2 without having played the original game, which is a shame since every game should have an entry-level opportunity. Not having a guidebook or walkthrough or any other reference could mean that I am doing certain sequences out of order (entirely possible) but by all indications, I am following the story line that the game has devised. Which leads to the first boss battle, Pete's Dragon, which was seriously challenging. There are several factors at play (movement around a circular field, spraying the dragon's skin, avoiding falling rocks and flames, while jumping over lava pits...) and the escalation of the battle doesn't seem to follow standard video game logic. In general, video game "bosses" follow a "three strikes rule" (you overcome them three times and you win) or are designed to strengthen a particular skill set. This battle takes on ALL of your skills and has several layers - painting the dragon's skin (which is a 360 degree process and maybe 15-20 areas?), then it respawns, then taking out the creature controlling the dragon, then moving up to a higher level where - again - you take on the dragon's skin, then the controlling creature inside and then... it just keeps going! Perhaps with a second player controlling Oswald, I might have fended off the dragon quicker, but it just seemed incredibly difficult for the "level one" boss. In addition to the big boss, there is a repeating clockworks enemy that looks like Herbie the Lovebug and seems to only be disabled or defeated by Oswald. None of Mickey's tactics seem to work, so extra effort is needed to herd Oswald to where you need him, in order to disable the car and then Mickey can paint/thinner the 'driver'. I like the idea of the cooperative play and that two are needed to defeat an enemy, but without having a human being controlling Oswald, these relatively minor baddies end up taking a LOT of time and energy to defeat (in addition to the number of times Mickey will 'die' during the process).
My favorite parts of the original Epic Mickey game were the sets and the music. The sets were so well crafted and areas like the Junk Pile were so fun to explore -- Epic Mickey 2 brings more of the same and even steps up the level of "set dressing" a little. However, there's a downside. Firstly, in the "underground" areas, you travel via a left-right scrolling game (similar to the projector screens) but the level of detail is so intense that it is difficult to tell where you're supposed to be or where to go. There is no clear "entry" or "exit" and I'll admit that I got stuck in the underground for a good 20 minutes. Take a look at this photo and see if you agree. The walkway that the characters are standing on is obvious enough, but can you jump on that turtle? The scary face on the right? The white circle at the end of the walkway?
In addition to being just plain confusing, the underground passages require you to set off a system of Rube Goldberg-type of contraptions in order to open a door or clear a pathway for movement. So you may end up climbing up vertically (or being tossed or sprung up there) to find a pinball in which to roll down a hill, which will knock down something, etc.
The music, again, is expertly executed and I honestly would purchase a soundtrack. However, in EM2, a lot of the soundtrack is lost by voice-overs. Every character that you interact with has an on-screen display (which you can turn off) as well as a spoken voice. This gets old FAST and is discouraging me from the side-quests, where you have to talk to multiple people to get clues. The voices are loud and "cartoony" (overly exaggerated bad accents) and everyone has a LOT of dialog. No one more than Gus, who is the Gremlin who accompanies you on your quest. In the original game, Gus was annoying because he'd interject here and there but didn't have an actual voice, just a grumbling noise which was bothersome. Now he speaks several sentences whenever he feels that you need to pay attention to something. His voice itself isn't bad, but about an hour in, I had to mute the TV because I couldn't listen to him anymore. As the quest continues, he seems to butt in less and less, but in the early phases? Ugh. Too much talking.
That being said, I really am enjoying the game over all. I wish that Oswald were a little more responsive or could be directed while in Single Player mode, but I'm adapting to working around it. I adore the sets and the "picture taking spots" are a fun touch. So far, my favorite is the Bog Easy level which is based on the New Orleans Square area at Disneyland. This game even has a Club 13 (a play on Club 33) which is so exclusive that not even Mickey and Oswald can enter! The different areas offer a lot of small challenges, which makes the gameplay even better -- it's not just chasing down the next bad guy, but also climbing on the sets and finding hidden treasures.