1-1: The hidden 1UP
I'd played the game a dozen times before I stumbled into this: an invisible block just a short way from the starting point of the very first stage, containing a mushroom that looked much greener than the others. I was just randomly jumping along when Mario's head smacked into a hidden block and the mushroom popped out. It startled me... and also launched a private crusade to leap at every possible moment on the off chance I'd find something cool (which really paid off once I got to World 3). Extra bonus: it forced players to make a tough decision -- go for the extra life, or duck down the pipe to collect coins a screen earlier? You couldn't do both, so you had to decide which bonus was more fulfilling. Me, I always go with the 1UP.
1-2: WELCOME TO WARP ZONE
After playing the game for a while, I started to see what kind of nonsense I could pull off. I'd noticed it was possible to run along the top of the stage in 1-2 if you could break through the ceiling (eventually uncovering another hidden 1UP in the process). But then I wondered, what would happen if I ran along the ceiling at the end of the level? Would the game explode? I wouldn't be able to reach the exit pipe... so maybe I'd be trapped in limbo forever. Needless to say, it was incredibly rewarding to discover the warp zone -- clearly, someone at Nintendo had predicted my game-breaking antics and decided to reward me for them.
8-1: Floor it
World 8 instilled me with an everlasting fear of endgames. It wasn't enough that the platforming challenges in the last three stages before Bowser's castle were insanely hard -- no, Nintendo had to go and up the stakes by providing barely enough time to clear the levels. No time for love, Doctah Jones. Or caution.
Everywhere: Sweet control
The best part of SMB, though, was something that permeated the entire game: a sense of total, perfect control. After years of games with simple, janky physics (not that anyone applied words like "physics" the videogames back them) the smooth precision of Mario was a revelation. He jumped low and high, but never felt floaty. He could -- quite unrealistically, mind you -- change direction in mid-air. Running added extra momentum to his leaps. And while he didn't stop on a dime, his quick skidding halts were consistent enough that any time you fell into a pit, it was your own stupid fault.
And that is why SMB rocks.