It was recently recounted to me that David Mazzucchelli, the incomparable illustrator of (among many other things) Batman: Year One, once commented that comics creators had put in a lot of work making superheroes more realistic and, in so doing, had made them less believable.
Fictional characters have a nature, both individually and collectively. Superheroes were conceived as fantasies, the pure embodiments of ideals (social and physical). Indeed, at the very beginning, they didn't really even have personalities beyond a commitment to that which they were fighting for. As they evolved, they effectively developed personalities and simulated psychologies, but somewhere along the way, it feels like a line was crossed (maybe Watchmen was that line), where the deeper into realism creators went, the further they got from the characters' natures. There comes a point where, when you try to explain and delineate the nuances of a fantasy, you merely highlight how much of a fantasy it is.