The number of available spaces for character names was increased from 6 to 9.
The random encounter rate was reduced in some areas
Diamond Weapon was made a into a boss rather than just appearing in a cutscene. This also made it possible to steal Yuffie's "Rising Sun" weapon, which was unused in the Japanese version.
The optional Emerald and Ruby Weapon bosses were added.
An FMV on Disc 2 was extended to show the new bosses.
The Underwater materia was made accessible.
Three items related to the Emerald/Ruby Weapons were added: The Guidebook, Desert Rose, and Earth Harp.
It was made easier to keep your body temperature up while climbing Gaea's Cliff.
The Temple of the Ancients' clock puzzle was simplified.
The mechanics of the Chocobuckle attack were changed from damage=(number of escapes)*(level of user) to just the number of escapes.
The animation for Safer Sephiroth's Super Nova attack was changed completely, bearing little resemblance to the original and being nearly two minutes long. Japanese Super Nova, US Super Nova. Additionally, the damage algorithm was altered. The original JP had a set spell power, while the US is (damage=current HP*15/16), or roughly 94% of a character's current HP, and it attempts to inflict multiple bad statuses.
A screensaver-like star field was added after the credits, similar to the end credits sequence of Final Fantasy IV.
The Test 0 enemy was removed.
Minor graphical changes to some backgrounds.