First appearance of Wario.
When Mario gets a Fire Flower, he wears a Feather in his hat. This was added since the Gameboy's system limitations (such as color), couldn't support the typical Color Palette of Red/White that Mario usually sports when getting Fire Power.
This is the first Mario game where the coin counter doesn't reset after 99. It continues to count up to the max of 999. You can use these in the Hidden cave just below Mario's/Wario's castle. There are 4 different games of varying coin amounts: 30, 50, 250 and 999, which can result in power ups, more coins, or lives.
When you kill 100 baddies, you'll get a free Starman.
If you press "Select" at the File screen, you can play the game in Easy Mode.