Although I will grant you that Superman has the highest name recognition among superheroes, I would also argue that he is among the least popular major comic book characters. His lack of popularity stems from the fact that he hasn't appeared in a successful, well-received film since the early 80s. For all of his name recognition, Superman Returns could only muster $391M worldwide. Even when adjusted for inflation that is still less than Thor made. In another thread it was pointed out that SR scored around the same as Thor on Rotten Tomatoes, 76% to the thunder god's 77%.
So even with higher name recognition and identical critical reception Superman could not earn more than one of Marvel's less known properties. Part of that is down to the quality of the film, which although well-received did not resonate with the audience the way many other superhero films have. But I would also argue that modern audiences simply are not particularly interested in Superman the way that they are in Iron Man, Batman, Spider-Man and others who are currently popular. The opening weekend total for SR was an underwhelming $52M, which is less than either Hulk film pulled in ($64M and $55M, respectively). (Even the lesser-known Green Lantern did as well in its opening weekend.) Hulk is fairly popular and well-known, but not as much as Supes, so that comparison is informative.
All of the above is to point out that Superman doesn't seem to enjoy the level of popularity many people assume he does. A Superman film starts off with an automatic advantage in terms of name recognition, but that will not necessarily translate into higher ticket sales. Better quality will help MOS, assuming that it is better than SR, but even that is no guarantee that the film will break into the upper echelon of superhero blockbusters, given the character's box office history.