The Doki Doki Panic conversion (SMB2 US) has always been my favorite Super Mario Bros game BECAUSE it was so different. Instead of "here's the Mushroom Kingdom.... scroll left, scroll left, scroll left... here's the next stage that looks the same..." there were worlds that actually changed and new gameplay elements that came into play as well as puzzle solving and boss fights that incorporated things learned up until that point in the game. It didn't feel like the same mechanic with a different skin or arrangement of blocks - the sense of progress was there. (again, something carried over into SMB3 as Jasoco said)

If you sit down and play Doki Doki Panic you'll notice that Nintendo reworked a lot of the gameplay mechanic as well when they did SMB2 as well as a lot of animation tweaking and changing here and there.

The whole reason I bought a GBA on launch day was to be able to play SMB2 on the go.

I've been playing the original Famicom Disk System version of SMB2 and while yes it's difficult that's really about it. If this would have come over to the US as SMB2 the SMB franchise probably would have turned into what Megaman is now days - a series that peaked with the release of the second game. *waits for Megaman fanboys to attack*