Nitsu: The difficulty wasn't manufactored. If you were good you didn't get hit. The difficulty is still avoiding all the crazy baddies in the first place.
The game had brilliant and large level designs, which were creative, challenging, and full of depth.
The mechanic to get Mario back I thought was imaginative, and added some challenge to the system yes, but mostly it added a depth to the concept of the game. And it was nice that hey you didn't immediately die hitting an enemy, you had a chance to recover.
I think that alone allowed Nintendo to really push the difficulty level up.