Official story and features:
"Fold Mario into a paper airplane and take to the air, turn sideways to slip through narrow passages or roll into a tube to bounce to safety in this exciting role-playing sequel. Many friends will join your party, and you'll have to rely on their skills to survive. Even former enemies may bail you out, so keep an open mind.
Take to the stage! Paper Mario builds on the action-packed battle system of the original RPG with the addition of audience participation. Not only will timing Action Commands help you dodge or inflict damage, but impressing the crowd will help you build power for super attacks! Try not to disappoint the audience, though, or you'll pay!
Don't fold under pressure! This time, Mario will have to make full use of his papery qualities to survive and there's no need to go it alone!"
From Nintendo site