That's not to say a Superman title should be a walk in the park; far from it. Note the recently-released movie, Superman Returns. Our cape-loving superhero is invincible for at least 135 minutes of the film, yet the film still has suspense. Why? Because Superman must save others, and that would be a great start for a Superman game -- no more "defend yourself" scenarios, but save as many people during a natural disaster as possible. If developers want a health bar, make it the current population of whatever location you happen to be saving. In this regard, a Superman game could be an intense, real-time puzzle, jumping from disaster to disaster, making split-second decisions, in order to save as many lives as possible.
Of course, there would be times when Superman would be up against a worthy foe -- he is allergic to Kryptonite, after all -- and in those cases a health bar is appropriate. But those fights do not have to be often. Shadow of the Colossus is a game with very few action sequences, yet each one made an impact because of the care developers took to making a boss sequences lively.
I am not a Superman connoisseur, but even so, I know the Man of Steel should never be taken down by a lowly thug. And that's the first rule by which every game designer should abide when contemplating a Superman game.
[Thanks for the 1UP feature, Erik Begley]
[update 1: As a few of you have pointed out, the Superman Returns video game, not yet released, does set itself apart from its predecessors by requiring more epic missions and putting the emphasis on Metropolis' safety instead of Superman's. However, the information on the title is still minimal and we will know more at a later date. Conceptually, it sounds promising, but the execution is not guaranteed.]