First off, A Link to the Past is not nearly the greatest Zelda game (Link's Awakening is). In fact, it's probably the Zelda game I liked least of all the ones I've played.
Those including every Zelda game except Oracle of Seasons and Zelda II.
It didn't feel like Zelda. It was just hack and slash bullsh!t; the puzzles were few, and simple. I liked the game, but it's not what I expect of a Zelda game.
Had to get that off my chest.
As for NSMB, collecting all the coins now. Have most of them. Challenging and fun.
With the exception of the special tube area in SMW, I'd say this is on par, difficulty-wise. Maybe I just didn't have that much trouble with the old games.
One thing, it is easier to get extra lives. But that's more convenient than anything, because I don't have to go through the Game Over, restart everything crap when I die. That's pointless, if I'm just going to start it back up again anyway.