The Other Darrin: All the returning characters, sans Ibuki and Akuma, got new voice actors in 3rd Strike.
Michael X. Sommers voiced Alex in the first two games, Patrick Gallagan in 3rd Strike (complete with a noticeable Brooklyn accent).
Bruce Robertson voiced Dudley in New Generation and 2nd Impact. Francis Diakowsky (the voice most commonly associated with Dudley) voiced him in 3rd Strike. Averted in the case of Naomi Kusumi, as he is Dudley's first Japanese voice actor.
Yun was voiced by Koji Tobe in New Generation and 2nd Impact. Kentarō Itō takes over for 3rd Strike and all subsequent appearances.
As for Yang, Wataru Takagi voiced him in the first two games, and Masakazu Suzuki assumed the role in 3rd Strike.
Michael X. Sommers voices Necro in the first two titles, Lawrence Bayne voices him in 3rd Strike.
Believe it or not, Dan was considered for Street Fighter III.
Sean was initially supposed to the only Ryu/Ken-type in Street Fighter III. If that would have remained, he may have still been wet behind the ears, but he probably wouldn't have been nerfed in 3rd Strike. Heck, he probably wouldn't be treated like a joke.
Gill's original design was to have him manipulate light and darkness. However, Capcom, for many reasons, decided to use the fire/ice alignment to showcase their spiffy new hardware. His secondary color is a reference to the original direction (but he still retains fire and ice control).
The dev team planned to include Hugo in New Generation, but he had to be Dummied Out due to time constraints.
Shin Akuma might have possibly been planned to appear in Third Strike, as he has nearly complete Dummied Out code in the game and can be completely playable with a hack.