about.com: The 12 Differences Between Super Mario Bros. 2 and Doki Doki Panic
When Mario 2 for the NES was released in the U.S., it looked and played differently than anyone had expected. It turned out that Mario 2 was not really a Mario title at all, but a game released in Japan as Doki Doki Panic. Nintendo had decided the actual Mario 2 was too difficult for an overseas release and pulled the title and modified Doki Doki instead. In order to make Doki Doki Panic Mario-like, they made some small changes, upgraded the character graphics, and sent “Mario 2” onto the market.
The details of what exactly Nintendo changed to create their blockbuster hit is somewhat of a mystery. Unless you’ve managed to find an original copy of the Japanese Doki Doki Panic, most likely you don’t have much idea of how much or how little Nintendo changed. [Full Article]