I used to agree with you, Smogen, but have you gone back and replayed the other Mario games recently? When I was young SMB, SMB3 and World all seemed hard, but they're a joke now and I can breeze through them like nothing. The only thing I still think is hard is SMB2J/lost levels because that game's just cruel. Even stuff in SM64 that seemed tough is pretty easy now, and I find I struggle most with awkward camera angles than any of the actual platforming mechanics.
As far as Casper being in charge of development, I find the custom of NSMBWii to be more intimidating than challenging. Especially since there are so many freaking enemies in 1-1 that it's practically impossible to come out with less than 99 lives.
Galaxy 2 is still pretty tough, it just hands out 1-ups like candy so you don't get too many game overs and give up. Personally, there were some levels that I died dozens of times before getting it right, even though some other levels I pwned on my first try.
The gist of what I'm saying is that I don't think that Mario games have gotten easier (with the exception of the NSMB games); I think that people like you and I have just gotten better with our platforming skills after beating the previous games so that all of the newer games seem easy. Watching my wife and son play Galaxy 2 makes me realize just how not easy that game really is. I'm just an outlier from playing the previous games way too much :-D