I can't beleive that a developer other than Nintendo could nail Mario so hard.
Anyway, while the other two Paper Marios were RPGs with a little bit of platforming, Super Paper Mario inverts that formula. While you still have HP, rather than battling for it in a turn-based RPG battle, you fight for it in a jumping-on-other-peoples-heads extravaganza.
This is where that little bit of RPG comes in. Not only do you play as Mario, but you also play as Peach, Bowser, and Luigi! They all have different special abilities, for example, Mario can flip into 3-D (more on that later), Peach can float on her parasol, Bowser can roast anyone who stands in his way, and Luigi can do a really high jump.
While you are teaming up with those characters, you will also meet pixls. They will give you different abilities and follow you around like parteners in the thousand year door.
The main hook of this game out of all of it, is flipping into 2-D and 3-D. The developers have pretty much made the whole game around that. There are some interesting puzzles where you will be in 2-D facing a solid wall, and flip into 3-D for fun, turns out there's a little hole in that wall.
This game is as charming as the others and I highly reccomend it to those who haven't played it already.