From TheCuttingRoomFloor:
http://tcrf.net/Super_Mario_World
You can see...
A cage has been left in the games code for mario to be in
Many overworld sprites were left out (Piranha plant, hand, Lakitu, Blue Jay, and a Koopa Kid)
Iggy Koopa would have had different hair in developement
The prototype title for SMW was "Super Mario World" with the subtitle
"Super Mario Bros. 4"
5ups are displayed incorrectly ingame
In the special stage "Funky" You get a message written out in coins "YOU ARE A SUPER PLAYER" seen here
http://tcrf.net/images/0/0f/SMWSuperPlayerUS.png
The same text appears in the japanese version with the text "YOU ARE SUPER PLAYER"
http://tcrf.net/images/8/89/SMWSuperPlayerJP.png