The famous SEGA jingle at the start of the game has been estimated to take up between 1/20 to 1/8 of the 4-megabit Cartridge.
Dr. Eggman's name was changed to Dr. Ivo Robotnik for all English language versions (see alternate versions). He would retain this name for all his English language appearances right up until Sonic Adventure (1998), when it was intentionally phased out and restored to Dr. Eggman, presumably because that game heralded Sonic's return to glory and there was a desire for him to return to his roots. However, the Robotnik name was officially acknowledged in Sonic Adventure 2 (2001), even in the Japanese continuum.
Sonic Team originally wanted to add a scene in the first level where Sonic was being chased by a large boulder, as in Los cazadores del arca perdida (1981). In the end, it couldn't be done, although (much like the object-throwing maneuver that had to be cut) this vision came to fruition years later with Sonic Adventure (1998).
At the time, everything about this game was revolutionary: the graphics, the sound, and (most importantly) the character. The idea was to make a game that would win over the Americans, with the hopes that this would guarantee success in Japan as well. While the game was indeed a runaway hit in most of the Western Hemisphere, it enjoyed only moderate success in the land of the rising sun, a trend which is present throughout Sonic's career.
One of the reasons Sonic can't swim is that Yuji Naka and other staff mistakenly thought hedgehogs couldn't swim.