I have two thoughts on the matter.
New Super Mario Bros Wii looks easy, but it does have it's hard spots. Watching someone almost always makes a game look easy.
3D platformers are easy. This is a fact of life. The reason is you typically are only threatened by missing a jump. Most bad guys can easily be avoided and they don't overload you with bad guys and platforms at the same time. In 2D platformers (like Mario anyways) they can put a lot of bullet bills, pits, and goombas and still have a challenging and enjoyable level.