Wario was so blinded with greed, he didn't bother making sure the game he was in was any good.
Past Wario games have been 2D action platformers along the lines of traditional Mario games. Master of Disguise takes a different route, adopting a style of gameplay more closely related to that of the Metroid games. There are areas that aren't accessible to Wario until he finds a specific costume. There are many different costumes, and each one has its own powers and abilities.
The basic costume is the thief costume, and this is used for agile movement, such as running faster and jumping higher. There's a spaceman costume that let's Wario execute floating jumps and shoot with a laser, a "shocking" costume that lets Wario illuminate dark passages, a dragon costume that has him spewing fire, an artist's costume that lets him draw blocks and hearts for health, a scientist costume that allows him to see hidden objects and passageways, a sailor costume that lets him paddle along the water's surface, and a devil costume that lets him fly. Each one of these costumes can be upgraded by finding the appropriate "Guise Gem".
In order to switch costumes, you have to draw a symbol on Wario. For instance, if you want to change into Arty Wario (the artist), you draw a canvas. To change into Captain Wario, you draw a prow infront of him. The problem with this mechanic is that the drawing recognition is horrendous. You really have to draw well in order for the game to realize what you're trying to do. Too many times, it mistakes what you were trying to draw. When you're in a boss fight that involves you changing costumes frequently, it can be a real nightmare. It's also a real pain to play as Arty Wario, because when you draw hearts for health, your hearts have to be almost completely perfect
The controls in general are just a real mess. The developers have forced the stylus on you, and for the most part it's really impractical. For instance, you have to touch the screen in order for Thief Wario to pull off a charging attack. When you want to enter a door, you have to touch the door icon instead of just hitting up, like how most games do. It's also very easy to tap yourself when attacking, causing you to accidentally turn into a costume you didn't want.
The microphone has also been forced on you. In order to fly as Wicked Wario, you have to blow into the microphone, and it's very hard to give him just the right amount of wind. There are several dangerous paths that you have to navigate through as Wicked Wario, and it's nigh impossible to do so without getting hurt at least once. The entire control scheme is a shameless gimmick, something that would have been expected back when the DS launched in 2005, but not two years later, when more traditional control schemes were proven to be prefered.
The level designs aren't even that great. Instead of trying to come off as clever and inventive, they just come off as annoying and frustrating. It really doesn't help that a lot of segments during these levels involve frequent costume changing. It makes going from point A to point B more of a twisted set of curves rather than a straight line. It also doesn't help that they're populated with uninteresting enemies.
Spread throughout these levels are treasure chests. There are three different types of chests. Red chests contain treasures that add to Wario's score for the level. Green chests contain the level maps, and purple chests contain very important items, such as new disguises, "Guise Gems", or keys. When you open the chests, you'll be presented with a mini-game. These mini-games vary from memorizing a picture, and then painting in the empty segments, to killing cockroaches, to tracing outlines while avoiding lasers. They get tougher as you progress through the the levels. If you fail to clear these mini-games, then a boobie trap is activated, and you have to start over again. They're fun at first, but after a while, they become old.
Normally, Nintendo games have great writing in them, the Mario and Luigi franchise being one such example, but the dialogue in Master of Disguise is just flatout atrocious. It's filled with lame puns and toilet humor, and it's clear that it's aimed more to the juvenile gamers. Sometimes, it can be humorous, but those times are rare. It's such a shame that it couldn't have been written better, because there are some lengthy dialogue scenes, and it just makes the game all the more annoying.
Graphically, this isn't a very impressive game. If it wasn't for the fact that Wario was in the game, you probably would mistake this for just another 2D platformer from some unknown company. The animations aren't particularly smooth, and the effects are downright drab. It just wreaks of lazy design and lack of imagination.
Audibly, it's not much better. All the sound effects are unimpressive. There's hardly any voice acting, aside from the occasional yelling of Wario. The sound track isn't very exciting. Some songs are decent, but others are just very poor. It just makes you wonder why a widely recognized Nintendo icon such as Wario was given such low production values when all his other games were of much higher quality.
This is by the far the worst Wario game available. His earlier 2D platforming games are worth hunting down and playing. His quirky WarioWare games should be experienced by everyone. Master of Disguise, however, shouldn't be played by anyone, unless you truly love Wario. It's riddled with problems, the worst being the drawing recognition. When your main gameplay mechanic doesn't even work half the time, it's a very crippling flaw. If Wario truly was a master of disguise, he would have disguised himself as a better game.