Minor tweaks in gameplay that were badly needed, but no real substance to this considering it costs money.
The balance tweaks fix old problems but introduce new ones; throwing fireballs from full-screen isn't as valid a strategy as previous versions allowed it to be, but Yun and Yang (both new to this version) are massively overpowered, particularly against projectile users, thus compounding the effect. The timing received a much-needed tweak that helped make the game feel a little more like Street Fighter of yore, and the replay channels are now actually usable. Most of the differences are very subtle, but make a significant difference in overall play to most long-time players.
Bottom line: My review sounds negative, but me talking about how great a Street Fighter game is would be redundant. If you know what cancelling is, you should probably get it. If not, buy Super; all the shiny new characters and ultras, but significantly lower in price.