The third game in the Paper Mario series was originally going to be a Gamecube game, but it has since been brought exclusively to the Wii. Since the game was brought over to Wii, it now uses motion control to enhance the experience. The main use for motion control is taking advantage of Pixel's special abilities. Mario can find Pixels scattered throughout the game, and each one has a different ability like dropping bombs or finding hidden secrets. The Pixels travel with Mario and he can switch between them at any time.
Also new to the series, you can play as Peach and Bowser at any time, switching between the three playable character at will, and each have their own special abilities. Mario's special ability is switching between a 2-D and 3-D perspective, Peach can glide ascross large gaps, and Bowser can use his brute strength and fire breath to punish his enemies. You will play as Mario most of the time however, as his ability is the most useful. For example, if there is a large boulder blocking your path in the 2-D realm, switch over to 3-D only to find that the rock is paper thin. This change between the dimensions opens up a lot of gameplay and creative puzzle opportunities.
This time around Paper Mario isn't a strict by the books RPG. Rather it's an action platformer with RPG elements. You can still level up your characters, use items, and deal damge to enemies. The battles now play out in real time, instead of turn based like the previous Paper Mario games. There are eight worlds with many areas to explore that increase the overall length of the game.