Street Fighter is a major exception because of the button combos. That's the whole point of the game, that these special moves require skill to pull off. But I could teach my grandma the basic controls of "these buttons are punches, these are kicks, up is jump, left and right move left and right, down ducks." Street Fighter is a complicated game so it isn't as easy to sum up as Mega Man but performing a kick in Street Fighter is easy.
Motion control is easier for people that decide for themselves that such-and-such is too complicated without making any effort to even try. My Mom won't play fucking PAC-MAN. The controls are just moving the joystick in the direction you want the guy to go. You can't make that easier but my Mom is just freaked of it. The ease of all these nifty doodads is an illusion. It's like the same logic used by companies that have those horrible imprecise voice activition call centers instead of the ones where you just push 1 for this, 2 for that etc. The voice recognition creates the illusion that it is easier for people unwilling to learn the button method but anyone who isn't close minded knows the button method is exact and simple and less frustrating to use.
Some games are going to be complicated to the point that some people won't play them and you can't explain the controls to them in 10 seconds (or for that matter the objective of the game). But those games will remain complicated regardless of whether you use motion control or not. But the games where it's just "move with the control stick, this is jump, this is shoot" are easiest to control with buttons.
I think the "sitting on the coach" factor also plays a big part in the ease-of-use. You don't need to clear space or set anything up and you can't get fatigued. I was playing Guitar Hero with my friends last weekend. I love it but setting up all the instruments and clearing the space for them is not something that I want to do all the time. Same with DDR. Any game where I have to move the coffee table out of the way goes from being pick-up-and-play to being an event. This is why I think controller-on-the-coach will be the norm forever because it's so low maintenance. It's like one step up from watching TV.