They are now able to add a lot more. Mario Kart DS was limited due to only having about 32mb available. Mario Kart 3DS will use game carts that can hold 2 or more GB, which is a LOT for a handheld system. So now, not too many limits. 
But the game is probably at its early stages of development, so expect the graphics too look a bit better when released.
But the game is probably at its early stages of development, so expect the graphics too look a bit better when released.