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
General
Another confirmation of more in-depth kart customization
Courses will break from convention (ex: Wuhu Island is just one lap, as it is a long course with numerous obstacles)