Superman's greatest power is his compassion. Throughout every successful iteration of the character that one virtue remains constant: he is an extremely powerful and endlessly resourceful man motivated by bottomless reservoirs of compassion to help people in whatever way he can.
This isn't a new observation, and it comes fairly close to what I think most people consider to be Superman's most basic core principles. But I don't think very many stories really take this idea as far as it could go. Certainly, Morrison's All-Star Superman is justifiably celebrated for being the best Superman story of at least the last decade, but its important to remember that the book succeeded not because it was in any way revisionist or "deconstructionist" (in the informal sense) but because it amplified the character's most central attributes to the point of bare iconography. It was in many respects the "purest" Superman story ever told, in that every story element was expressly dedicated to reflecting some facet of Superman's core thema. It is not the type of Superman story one can imagine coming across very often, because the tone is so unabashedly sincere that it would probably seem merely bathetic in the hands of an inferior creative team. Despite whatever qualms I may possess in regards to latter-day Morrison, there's no doubt that All-Star Superman is a towering work in the field.
But it wasn't All-Star Superman that inspired me to muse on this subject, it was a far less celebrated spin-off limited series from the mid-90s called The Doomsday Wars. If you don't remember it, don't worry, it's been largely forgotten for a number of reasons - the first of which being it is deeply mediocre, and the second of which being it served as a prelude to another in a long line of subpar Brainiac revamps that stretched from the immediate aftermath of the Crisis and on through very recently. I reread the series recently on a whim, looking for a light read and vaguely remembering the series (along with its predecessor, the actually-pretty-decent Hunter / Prey) being a good popcorn read. Sure enough, the actual plot was not particularly memorable, but there were a few bits that did stick in my mind. There's a subplot involving Superman remembering a story from his youth - mid-teenage years - wherein, during a fierce blizzard, he was unable to reach a herd of cattle stranded on a far field, and they died because he crashed the truck into a snowbank while trying to reach them. (Keep in mind this was still the post-Crisis period when Superman's powers did not even begin to emerge until late adolescence.) The flashback echoes the contemporary story, with Superman trying to carry Lana Lang and Pete Ross' newborn son from Kansas to a state-of-the-art neonatal care ward in Atlanta, but being waylaid by Doomsday in the process. (Don't worry, he saves the kid, but not before getting the snot beat out of him a few times. It doesn't end on a downer.)
The point of the story is an important one, despite the rather gruesome imagery of a young Clark Kent being traumatized by dozens of dead cows buried in shoulder-deep snow. Every now and again someone does a story that follows the general idea, "Superman can't save everyone." It's a downer, yes, and there are certainly many examples of the trope done poorly - but it's necessary to do the story every now and again for the simple reason that it underscores what might be the character's single most crucial character trait, the one virtue that enables everything else he does: humility. He is (for all intents and purposes, Captain Marvel notwithstanding) the most powerful man on earth. And yet he must be constantly aware of his own limitations, always conscious of exactly what he can and cannot accomplish with his powers. He knows that there are many, many things that he simply can't do even with all the power in the world, and although it might prove frustrating time and time again - and provide fodder for countless stories - at the end of the day he is Superman because he accepts these limitations and moves forward to do the best that he can.
He has to be able to forgive himself for not being able to be everywhere and do everything, and so by necessity he also has to be forgiving of others as well. Few writers have spent time articulating just how differently the world would seem to someone like Superman. His senses would give him an unenviable vantage point from which to observe humanity. Even if he never used his hearing or his sight to invade privacy - which would probably be fairly difficult to do in absolute terms - he would still be privy to more of the panoply of human behavior than anyone other being in history. He could see cause and effect, the roots of poverty and wealth, the consequences of charity and compassion. Elliot S. Maggin's averred that Superman would have to be a vegetarian, because his enhanced senses, extending to the infrared spectrum, would enable him to "see" the heat auras of living creatures, and register their emotions in much the same way as Daredevil does. He couldn't eat meat because - having grown up on a farm - he would be intimately aware of just how much pain an animal suffers as it dies, would be able to feel, see, smell and hear the process so viscerally that it would be overwhelming.
I think if you extrapolate that idea outwards, it's not hard to see that Superman's compassion is completely reflexive and therefore completely inextricable from the character. It's easy to do an "evil Superman" - just give us the same basic person with the same powers only without the compassion. Without that bedrock human decency, it's hard to see why all that power would not corrupt - but if you believe that "super empathy" is as much a part of Superman's powers as super strength and hearing, it's easy to see why the character would remain so steadfast throughout decades (and, in many alternate versions, centuries and even millennia) of the "Never Ending Battle."
To the best of my knowledge Neil Gaiman has only written one Superman story (not counting cameo appearances), the Green Lantern team-up Legend of the Green Flame. Originally written to run in Action Comics Weekly (that was a long time ago), it was dusted off and finally published in 2000. It's not that memorable of a story, but there's one bit that's always stuck with me. The gist of the story is that, thanks to a mystical MacGuffin (something to do with the Golden Age Green Lantern's lantern, considering that this story was supposedly set during the period when the original Justice Society had been exiled to fight an eternal Ragnarok inside a pocket universe [an odd Roy Thomas plot that was also mentioned during Season of Mists]), Superman and Hal Jordan are killed within the first few pages, and spend the rest of the story wandering the afterlife trying to find out how to return to life. There's an absolutely great bit with Superman and Hal in Hell - the real Hell - and Superman is rendered almost completely insensate. His can see and hear everything, and it's impossible for him to look away from the limitless catalog of torture and suffering in the inferno. He just stares, eyes wide open, unable to do anything but float rigidly above the lake of fire. When faced with the apogee of human suffering, suffering which he is definitively incapable of alleviating, then and only then when hope is obliterated can Superman be completely defeated.