It's nothing you haven't seen before, save for a giant Boo and some co-op gameplay / sibling rivalry.
New Super Mario Brothers 2 holds the special distinction of being the first ever fully downloadable game on the 3DS (or any Nintendo system), as well as the first Nintendo game to feature DLC. Theatrhythm Final Fantasy was the first third party game to do DLC on a Nintendo system this month.
The game has an obvious emphasis on coin collecting, with various items dedicated to producing large amounts of coins. These include Gold Flowers that turn blocks into coins, rings that temporarily turn enemies gold and a block mask that produces coins as Mario runs, as well as a gold variant of the Super Leaf.
The game also features a Coin Rush mode, where players can play through a set of three levels collecting as many coins as possible with only one life. Players will be able to challenge other players over Streetpass, or play co-op if you're not the competetive type.
Give it a try on the new 3DS XL, which is out right before New Super Mario Bros 2's release in mid-August.