The subplot about Dr. Wily being Dr. Light's old assistant and turning on him only exists in the American version. In the Japanese version, Wily was just a random mad scientist who had no initial connections to Dr. Light prior to stealing and reprogramming his robots.
The only version to keep score.
Even though Roll appears in the ending, her name isn't given in an American game until Mega Man III (NES).
In Korea, Rockman was once known as "Mecha-Man" to avoid lawsuits with the creators of a 1970s Korean TV show that was named "Rockman". But later, eventually, most versions released throughout Asia all became known as Rockman, because that title was more well known than "Mecha-Man" or "Mega Man".
Some differences from this game include power-up life and weapon energy capsules looking different than other games, items reappearing if you leave the screen and come back, no spike invulnerability when hit, and the keeping of score. Most of these issues were fixed in later issues, like the Japanese-only Complete Works reissue, and the Anniversary Collection.