Superman is highly aware that most of his opponents are often less durable and usually less physically powerful than he is. When in doubt he uses the least amount of force required to handle his opponents. To understand his actions, they have to be seen in context.
The fight opens with Superman using his super-senses to detect that Batman's utility belt is empty. This means he is always scanning his opponents. Something prudent when you aren't sure of the capabilities of your opponents. He does this to everyone he fights.
While the Flash's initial interactions befuddle Superman (he likely has fought few opponents as fast as he is) once he realizes he can match his opponent's speed, he restrains himself, pulling his punch when he having taken the Flash's measure sees he need not use more than the tiniest application of force. Your description might have seemed to adequately describe the scene but in context it appears more as Superman getting the measure of his opponent.
Superman always engages in restraint during combat. Always. With strength as great as his, it is important for him to exercise control because he is morally opposed to killing his opponents. In this fight he applies the force necessary to damage his opponent, nothing more. Check out his eyes, he is predicting where the Flash will be and times his blow to make sure he is where he needed to be. Once he was sure he could hit him, he instead just flicks him. That tiny blow knocks the Flash back down the street.
Though they have not shown it in the DCnU, in the previous continuities, Superman was in the habit of scanning his opponents to determine how much force to use in combat. He uses his superhearing to detect heart rates, his x-ray vision to determine physical bone density. He also takes note of their ability to deal damage to him since if they are strong enough to hit him and hurt him they should be strong enough for him to use more of his capability in a fight. Even if he believes his opponent is strong enough to handle the full out use of his powers, he will still restrain himself in order to reduce collateral damage to the environment and the people who may still be nearby.
A quick survey of the Flash's physiology revealed, despite his amazing speed and ability to deliver super-fast punches, the Flash was in no way capable of surviving a full-on direct punch. A flick of the finger is more than adequate for knocking out someone whose overall physical capabilities is only a little more than a highly trained human being.