There are so many differences in a punch. There are huge differences between punches from Muay Thai versus a Wing Chun punch. Or between a krav maga jab vs. a brazillian juijitsu cross. Even between similar styles... for instance a Wado-ryu fighter with take a secondary lunge step for a cross punch where a shotokan fighter will plant off the jab, and throw their cross. Minute differences can be seen in foot, leg, hip and shoulder rotation, elbow angles, path, striking surface, position of each shoulder, timing, rotation of arm, power, speed, direction, angle, and last but not least, technique. So to say a punch is just a punch, is like saying a fish is just a fish.