It contains a tilt motion sensor simmilar to the iPod/iPhone, and the Wii Remote, and will feature a virtual console of Gameboy & GB Color games, in addition to simple classic games, (I'm guessing, Puzzle League, NES, and a few simple GBA/SNES/Sega platformers)
I hope Original Gameboy games are colorized, with an option to switch between colorized versions, and original color palettes. With wirless Multiplayer for all Multiplayer games.
They also confirmed a "Mii Studio" to create, & edit Miis,
given the name, I'm guessing You can alter the Mii's appearance even more, and extra data will be unused when Miis are transfered to a Wii.
I read somewhere it takes 3D video as well, but it might just be 2D video with 3D pics, I don't know.