The Other Darrin: Knuckles is voiced by Scott Dreier in this game, when he was voiced by Michael McGaharn in the first. However, their voices are similar enough, so it's hard to notice the change.
Tails, who was voiced by Corey Bringas in the first game, is now played by Corey's younger brother, Connor.
What Could Have Been: Originally the game was only going to feature Sonic, Knuckles, and Eggman as playable characters. The alternate stages would be branching, a feature that would return in Shadow the Hedgehog.
Shadow’s original name was “Terios”, which means, “reflection of”, and Rouge’s original name was “Nails”
Sonic was originally going to wear his sneakers from the Genesis games, instead of his SOAP shoes which enabled him to grind.
Some leftover data from the Dreamcast version shows that originally Chao Kindergarden was going to be in Sonic Adventure 2, instead of Battle.