New racers

Lakitu (kart looks like the cloud he uses)
Metal Mario (shiny, silver kart)

New modes

Race in first-person using the gyroscope
Tricks system like Mario Kart Wii

New power-up

Fire flower (thrown to the front and rear of a kart)
Opponents will spin if hit by the fire flower

Online

Up to 8 players
Local multiplayer as well
StreetPass support – Trade ghost data directly with friends; register others through StreetPass passively so you can play them online later
SpotPass support – Receive ghost data from others around the world

Other information.

Courses will break from convention: example –  Wuhu Island is just one lap, as it is a long course with numerous obstacles

Link