SFII:CE Rainbow Edition added some nice features. Although some of the features could be abused quite easily(!).
Ability to change characters while in play, by hitting the 1 & 2 Player start buttons.
Players have different behaviours and speeds for their projectile weapons. They can also have more than one of them "active" at a time.
Players that did not have projectile weapons now have one.
"Air Throws" can be executed on players that are on the ground.
Many "Special" moves can be executed while in the air.
Some players have faster moving speeds.
Fun to play with the computer, but tiring with a (over-cheap) human opponent.
The game's AI is not designed to handle some of the modifications that Rainbow Edition includes. One of them being multiple projectile weapons being active at one time. So if you throw 4 (slow) "Sonic Booms", the computer will block the first one, and then will proceed to walk into the next three!