One of the bigger mistakes (among many) that the people at DC have made in the last few years has been to almost completely disregard Clark Kent. One of the very best things to come out of Byrne's Man Of Steel revamp was his reconceptualization of Clark Kent as more than just a mask for Superman. The idea of Superman as someone fully in touch with his Kryptonian heritage and slightly removed from the run of humanity ran its course in the years leading up to the original Crisis. Introducing a Superman who never even heard the word "Krypton" until (I believe) his late teens meant giving us a Superman who believed himself to be fully, completely human in every way that mattered.
This might seem like picking nits - after all, Superman is Superman, right? - but it goes back to the compassion at the heart of Superman's character. Despite his great powers Superman really does not - cannot - believe that there is any meaningful difference between him and any other other person on the planet. He truly believe that the only thing separating himself from the run of humanity is that he has been given by accident of birth the opportunity to act on his compassion. And therefore it is necessary for him to believe himself human, to believe that as much as he is Superman he is also a man named Clark Kent who uses the means available to him in order to fight for the same ideals that Superman holds.
The biggest mistake any writer can make is to portray Superman as naive. Superman is an optimist, yes, and an idealist. He honestly and genuinely wants to see the best in every person he meets. But Clark Kent is a journalist. As a journalist he is necessarily well-acquainted with the absolute worst humanity has to offer. Different creators are inconsistent as to exactly what kind of journalist Kent is - specifics don't really matter so much. Journalism is essentially a good plot device to enable Kent to be put into a number of different situations. But whether he's walking the political beat, doing crime or business or war or even sports, Clark Kent is constantly being pulled into close contact with bad people doing worse things. Whatever kinds of corruption and cruelty might miss his eye as Superman, he sees as Kent. He's a trained investigator with a super-brain, able to sift through massive amounts of data in the blink of an eye, and probably about as shrewd and clever as he wants to be depending on his circumstances. His limitations as Kent are that his reporting is obviously limited to what he can verify as Kent with the cognitive faculties and resources of a normal human at his disposal. (It strikes me that there is a great deal of story potential to be found in the discrepancies between what Superman can know and what Clark Kent can prove.)
So it's probably fairly hard to surprise Superman. He's seen it all, either as a super hero or a journalist. And that is one very important reason why Clark Kent is so necessary to Superman: without Kent, it's easy to lose sight of how smart Superman is. He is very smart. He may not have the same instincts and specific detective experience as Batman, but he's usually able to suss things out just fine on his own. It's important to Superman not to be seen as particularly calculating or cynical - because he's neither of these things, not really. But he can be these things, and being Kent is necessary because it allows the reader to see that the character understands shrewdness and cynicism just fine. Most of the time Superman can't afford the luxury of being cynical: he has to be perceived as eternally optimistic, because that's where his power lies. He is eternally optimistic, but it's not because he's ignorant of the "facts" regarding things like recidivism rates and political corruption and ethnic cleansing. It's in spite of these things that he carries on in the face of the worst the world has to offer.
If you wanted to make the comparison to Batman, this is where the two characters differ. It's Clark Kent's job to acknowledge the hard facts and expose malfeasance, to be suspicious and to act on those suspicions. Superman presents another option: an alternative based on forgiveness and an ideal of mutual responsibility. Bruce Wayne, as a public philanthropist and not-so-public captain of industry, uses his great resources to work on the larger scale for hopeful outcomes, helping ex-cons get back on their feet and funding educational initiatives to lift the working poor out of the desperation that begets crime. But as Batman he deals with the failures of the social welfare system and is forced into close contact with the scum of humanity. Superman and Batman are both essentially two people working for the same goal, but the genius of their dual identities is such that they are each able to become their own compliment, should the need arise. It's moot since (in most incarnations) Batman and Superman always know each other's identities, but it's conceivable that a Clark Kent / Batman team up - two investigators using their minds to expose corruption - might be almost as interesting as the traditional light / dark dichotomy of the World's Finest team.
Garth Ennis is rightfully praised for Hitman #34, featuring Tommy Monaghan's first unlikely encounter with Superman. Rarely discussed is the follow-up, 2007's JLA / Hitman, a direct sequel from the first story picking up the thread of how exactly Superman would react once he knew that the man with whom he had shared such a powerful moment was, in fact, a hired assassin. Everyone in the JLA reacts to Monaghan with the same identical abhorrence - he's a killer, a mercenary, a murderer. But Superman . . . Superman can't forgive Tommy for what he's done and what he does, but by the same token he can't bring himself to condemn anyone - even a hired killer with hundreds of lives on his hands. It's not moral cowardice that compels him to great benevolence even towards his enemies, rather, it's his limitless compassion that enables him to perceive the best in even the worst specimens of humanity.
There was a truly great moment in the otherwise underwhelming conclusion of Paul Cornell's recent "Black Ring" arc in Action Comics. At the climax of the story, after chasing a serious of MacGuffins halfway across the universe and coming into contact with some of the most terrible villains in the universe (as well as Death), Lex Luthor has achieved his lifelong goal of godlike power. He has the ability to do anything - to remake the universe in his image, to create lasting prosperity, eternal peace and harmony. He even wants to do these things - but there's one thing he wants to do first and more than anything else. That's right: kill Superman. The only problem is, because of how the power works, it will only obey him if his actions are completely benevolent.
It shouldn't require any kind of spoiler warning to tell you that, of course, Lex is unable to overcome his animus against Superman, even at the highest cost imaginable. But what's Superman's first instinct, when face to face with a godlike incarnation of his single greatest enemy? Forget me, forget everything about me, he says, I'm not important, I'm every bad thing you've always said I am. Just put me aside and do something good.
Because that's what Superman is all about: everyone, even Lex Luthor, is as capable of doing good as anyone else. We all possess within us the potential to perform great acts of kindness, because simply by being human we have been given the ability to choose good over evil. Even if he's 99% certain that a person will do the wrong thing, he has to believe that, if given the chance, anyone will be able to rise up and better themselves, to be that 1% that bucks the odds and makes the world a better place. If he didn't believe that with every fiber of his being, he wouldn't be Superman.