Both GH and WiiFit appeal to casual players. GH on Expert is quite difficult and that challenge appeals to hardcore gamers. WiiFit may get difficult we just don't know yet.
They are both less innovative then people most people recognize.
Guitar Hero is a popularized version of http://en.wikipedia.org/wiki/Guitar_Freaks from 98
WiiFit is a popularized version of http://www.yourselffitness.com/ with a scale controller added
There is nothing wrong with not being the first game to do something. In general it's the first game to do it right that makes the genre popular. GuitarFreaks had a horrible song list for western gamers and WiiFit will make it so your progress can be tracked on screen instead of entered through the UI.
The big difference is the experience the simulate.
GH allows you to be a quality guitar player in a band rocking out in front of an audience. That's a fun fantasy to fulfill that isn't easy acquire in real life. I've had a couple guitars for years, but I'm not in a band and certainly can't play at the level to complete a lot of the songs in guitar hero. So the game is letting me do someone that otherwise would be out of my reach without a ton of dedication. Additionally those guitars and amp cost me more then guitar hero did.
WiiFit allow you to do a bunch of things you could do cheaper without a console or the game.
Stand on one leg? Yep I can do that.
Hula Hoop? $0.99 at the dollar store and I can try that as well.
Basic Line dance? $9 videotape
Yoga? $9 videotape
Play Soccer? $20 for a ball and walk to the park down the street
WiiSports was fun because I don't have a bowling alley in my house, and it gave me cheap access to one. What WiiFit is simulating is easier and cheaper to do without the game. Thus it is open for parody and derision: http://www.youtube.com/watch?v=_iYBmAVuBns
