One idea you couldn't do in SMB1 was going backwards. SMB1 was a game that was going to be included with almost every console that was sold so the cost of the cartridge really mattered to the corporate bottom line. Not only was the rom chips for the SMB1 cartridge a fraction of the size of SMB3 (and a fraction of the price), but SMB1's cartridge contained no expensive SRAM chip.
On SMB3 you will find the level data stored at 6000+ which is inside the SRAM. This level data is what allowed you to go back to areas in the level you had already played and find consistency. In SMB1 you couldn't go back because it didn't store that data and so the bad guy you killed and the blocks you destroyed would have to appear again.
The tiny SMB1 board: http://blog.grunick.com/wp-content/uploads/2014/06/IMG_9484-...
On SMB3 you will find the level data stored at 6000+ which is inside the SRAM. This level data is what allowed you to go back to areas in the level you had already played and find consistency. In SMB1 you couldn't go back because it didn't store that data and so the bad guy you killed and the blocks you destroyed would have to appear again.