2 things about SMB.

1) The physics. All of the objects interact in a consistent way. At first there doesn't seem much to the game but the relatively small number of objects can all interact with each other. Seemingly simple things like being able to change the direction of a mushroom by hitting the brick its about to move on from below for example. The fact remains that the gravity and inertia effects on the control are sublime - and unmatched by any dodgy copy - just look how floaty the Giana rip off looks compared to the original........

2) The level design. Every enemy and hole has been set in JUST the right place - who knows how many hours of playtesting each level had. The fact that there are other things to do besides find the end too - the flagpole, the coin collecting (for the time a brand new mechanic), point chasing by following a moving shell at just the right distance, the beanstalks and warps to find, figuring out the fireworks bonus. All excellent stuff and it goes together to make the game still playable to day. I played the original to death but I prefer the All stars version on the SNES.