The Wart Boss Fight was originally going to sport tomato projectiles like its original incarnation in Super Mario Bros 2 featured. They were replaced with cabbages in the final release, but the tomato sprite is still present in the game, albeit unused.
When the original Super Mario World game was released, Luigi was a recolored copy of Mario. In Super Mario All-Stars + Super Mario World, Luigi was remade to look different to distinguish the two.
There's an unused fanfare for Super Mario Bros. 2, not included in the NES version. It was eventually used as the Spade/Heart/Club House victory jingle in Super Mario Advance 4: Super Mario Bros. 3.
Right before the "lights" come on and illuminate the title screen, the crowd of Mario characters can be heard talking. Even though you can't really make out any words, the crowd chatter sound clip was changed for the U.S. version.