In a short answer... Everything.
In a long answer... The controller (more to come shortly), the graphics, the third party support, the possiblities of gameplay, the creativity (I am a creative person who loves drawing and painting), the fact that it is made by Nintendo, the games we KNOW will be on the system (Zelda, Mario, Metroid, among others), that they are continuing the ability to use the wiimotes with motion plus and things since it reassures the value of buying motion plus accessories and other things, backwards compatibility, possiblities with online once we learn more about it, the more hardcore approach while maintaining games that appeal to the group (family and friends) environment, the social aspects of the controller (video chat), 3DS connectivity, what sounds to be a strong line-up of games for release and post-release, I know I said graphics already but by all that is holy the Zelda and japanese garden tech demos were beautiful, being able to stream from the TV and use it as a semi-portable device, AND the fact... that there is more to come that we are still unaware of.
This is what I like about the system, I am not trying to make anyone interpret what I am saying as anymore than my opinion. I am drawing what I like about this system, based around what I, myself would find useful and convenient.