Playing as Bowser is the main addition, but there is plenty of other stuff to do for hours of gameplay
The story is different from your average "Peach gets kidnapped, Mario rescues her" storyline in most Mario games. Instead, the story goes that a disease called "The Blorbs" is affecting the Toads which inflates them into a massive ball, so Peach holds a meeting to try and help them. Bowser then eats a mysterious mushroom which makes him inhale things around him. He then goes to Peach's castle and accidentally inhales Mario, Luigi, Peach, Starlow (a star sprite), various Toads and a few warp pipes. It is quite a deep story with genuinely funny writing.
The main attraction here is of course being able to play as Bowser himself. Not long into the game you gain the ability to inhale enemies as Bowser. You then continue the fight inside him as Mario and Luigi. Bowser's moveset contains the things you would generally expect him to do. He can punch and breathe fire and use special moves with his minions. Battling consists of selecting your move and doing a timed button press to add power to your attack. It is fun and rewarding and you'll find yourself initiating battles just for the fun of it.
Overall, this is a brilliant must-have game which will please Mario and RPG fans alike.