According to Capcom's art director Toshiyuki Kamei, Hakan's turquoise, stone-like hair is a kind of pun. In Japanese, Turquoise is "toruko ishi". While "Ishi" can mean "rock" or "stone", "toruko" can also mean Turkey. This is a nod to the fact that Hakan is Turkish.
Abel was originally female during the development of the game. Instead of traditional MMA style clothing, the original Abel wore a Kurateka (Japanese wooden footwear). A black version of the said Karuteka was given to Ken as an alternate costume, but the character was ultimately rejected.
Rufus' original concept art is completely different when compared to his final design. He was shown to be much slender, African American and wore martial artist robes with the words "King Cobra" on the belt. The final design was chosen because the developers wanted to leave an impression on players with his obese appearance and set him apart from the other characters.
When you beat the car smashing bonus stage with either Cody or Guy, a man will side step into frame, drop to his knees, and yell, "Oh, my car!". This is a reference to the car smashing bonus stage in Final Fight, where the same thing would happen if you completed it in time. This scene can also be triggered by all of the Final Fight characters in the USF4 version of the game.