The main upgrades are the 3D, which I can't actually use based on poor eyesight. However as the 3DS is the new iteration of the nintendo handheld, you can't play new games on the old consoles.

3DS brings about streetpass, which you can see here http://nintendo.wikia.com/wiki/StreetPass. It allows for the communication between 3DS automatically if they are in range to trade data between games. Pretty cool function actually.