I am out of words.
Well I guess I should say that I think the original Super Mario Bros is a true masterpiece, still one of the best plattformers ever, brilliant in both level design, challenge and controls. However, as with every action-game, it does kinda defeat the point when there is no longer any challenge. As when you use save states at the start of each level. Then you will basically be able to "grind" through the game.
Also, there is unlimited continues in the game. Hold A when you die, and you will start in the world you died in (but not the sub-level). There is also warp-zones to quickly take you back to whatever world you where in.
Remember, this game was released when single-screen gaming was all the rage. And it was the single biggest leap forward for gaming ever, almost single-handedly taking the video game industry out of the slump. Super Mario Bros is quite simply, the most influential game of all time, and still maybe the best.