I managed to have play on a 3ds last week, very interesting.
The 3d is kind of like playing a game through a video camera, it gives a great perspective of depth and also you can turn around and look at things from different angles etc, its quite impressive but obviously there's no objects jumping out at the you or appearing beside you, like the cinema? ( i don't know, never seen a 3d film!)
I think in terms of mk, it will almost be like playing from the perspective of a camera following the car, like the replay views in mkdd maybe?
p.s. in a perfect world the game would include jumping/hopping, no-bikes, quick mt's, not too many retro courses and no thunderclouds/bullets in multiplayer imo!