On the whole, The Thousand Year Door plays a lot like the original Paper Mario. Battles are turn-based but feature timed button presses and other similar player input mechanisms that can increase damage on offense and reduce it on defense. Enemies are visible on the screen, which means that there are no random battles. The game does contain some platforming elements, as well as badges that the player can equip to add special attacks, boost stats, or provide other similar effects. The game is broken down into multiple chapters, and while most of it features Mario and the companions that join him along the way, there are also short bits starring the abducted Peach as well as Bowser, who is on a rampage trying to find both Mario and the people who kidnapped the princess in his stead.
Of course, there are some notable new additions. Most noteworthy is the addition of different "curses" that will befall Mario, which are actually creative new abilities that allow him to change into a paper airplane, a boat, etc. These are used to solve various puzzles along the way and, given the whole "paper" theme of this series, they fit nicely into the gameplay. Also, some of the chapters feature unique scenarios, such as Mario assuming the role of a professional wrestler or being forced to solve a mystery on board a train. Also, all of the battles take place on a stage, and how well (or poorly) a player performs at combat will result in an audience forming or dispersing. They are essential as Mario will eventually gain powers that can only be recharged through the cheers of those in attendance. The mix of the familiar with these fresh elements works well.