You'll be able to collect data from other players by passing by them, using StreetPass. You'll pick up win/loss records, coin data, and more, including "ghost" data of other players' races, which you can then race against. You'll also have the option to install a Mario Kart Channel to your 3DS, to act as a hub for downloaded Mario Kart 7 stuff.
Nintendo also announced a couple new playable characters: Lakitu and Metal Mario. In other news, Nintendo is apparently running out of ideas for Mario Kart characters.