If he put all of his might in it, the punch would obliterate everything around you
Man of Steel opened this weekend and, if you've seen it, you've probably noticed him punch quite a few people in it.Even if you haven't seen it, you can safely bet that punching people, human or not, happens at least once in the entire movie.
Comic book physics, movie physics, and actual physics have very little in common. And that's definitely true in the case of Superman.
A video by Vsauce describes what would happen if Superman punched you in the face. TL;DR, you'd be disintegrated, and your atoms would be disintegrated as well, along with everything else around you at a range of about one kilometer (0.6 miles).
That's, of course, if Superman would put all of his might into that punch so that it would reach very near the speed of light. At that speed, a fist would contain as much energy as several thousand nuclear bombs.