First and foremost: You gotta download the update.
So, what's in the update? 1. The big new character, Piranha Plant from the "Super Mario Bros." series.
Here's a look at Piranha Plant in action:
2. A few new additions to the game's single-player story.
3. One major gameplay change: Pushing both jump buttons will make you "short hop."
4. A ton of little changes to the game's many, many characters.