(or, "The Gold Standard")
Obviously, the status quo for Superman is a wonderful storytelling engine. You can tell because he's been around for some 80 years in continuous publication, which clearly indicates that a lot of people have been able to find good stories to tell about him. But the first thing to nail down when discussing Superman as a storytelling engine is, "Which Superman?" He's a character that's been reinvented and refocused any number of times, and the character Siegel and Shuster wrote about in the 1930s is not quite the character that John Byrne wrote about in the 1980s. Since we're looking at these characters through the lens of "DC Showcase Presents", we'll be looking at the Silver Age Superman of the late 1950s and early 1960s, also known as the Mort Weisinger era after its editor.
One thing to keep in mind, we're looking at these stories as a measure of how easy it was to find things to do with Superman, not how good the actual stories were. Silver Age Superman stories did tend towards the bizarre and contrived, sometimes brilliant and sometimes terrible, but the amazing thing about them as far as we're concerned is just how easy it was to find stories to tell. For starters, the basic concept of Superman is practically archetypal: "Last child of a doomed alien race raised by humans and possessing super-powers" is the concept for dozens of other characters out there. It's difficult for us to even imagine a world where that was a new idea, it's so common in popular culture; it's an idea that has roots in mythology and folklore (substitute "gods" for "a doomed alien race" and you've got Hercules), but Superman is in many ways the definitive "Last Son of (Blank)".
"(Blank)" is, in this case, Krypton, and Silver Age Superman actually uses the planet as a frequent backdrop to Superman's adventures. Establishing time travel in the series allows return trips to Superman's lost home planet, giving writers the chance to do more explicitly science-fiction stories and show an alien world. (Krypton is very much a classic sci-fi utopia before its eventual end, with flying cars, bizarre alien animals, and careers decided by computer.) Krypton is also represented in the present-day by Kandor, a "city in a bottle" shrunk by alien science that Superman can visit, which gives writers a chance to do more Krypton stories and gives them a source of other Super-people whenever they want to have a convenient "Superman vs. someone just as powerful" fight.
Kandor is, in turn, just one of the story options available by setting a story inside the Fortress of Solitude; Superman's all-purpose lab, trophy room and secret headquarters contains an almost innumerable array of alien technology, lost artifacts from different civilizations, and essentially serves as a story generator all by itself. You can always find something in there for Superman to do for 8 pages (Superman comics in the Silver Age tended to have two or three separate stories in them, a concept almost unheard of in today's "writing for the trade" era.)
Completing the circle for Superman is Metropolis--if Krypton represents his alien heritage, and the Fortress is a piece of his alien world on Earth, then Metropolis, and more specifically the Daily Planet, represent his human side. Here, we find no shortage of human crime (although Superman's globe-spanning speed lets him find trouble anywhere on Earth if the writer wants to), and more importantly, we find a supporting cast that gives yet more storytelling opportunities to a potential writer. Lois Lane alone seems capable of generating dozens of Superman stories; she's a crusading reporter, a potential love interest, and a curious woman with an interest in Superman's real identity. Any of those three traits give Superman a potential point of entry into a story. Jimmy, in turn, is a bright young reporter with a nose for trouble and a watch that lets him signal Superman. Again, this is enough to sustain its own series (and, in fact, both Lois and Jimmy have had their own series in the past, which we'll be looking at later on.) Combine them with Perry as the gruff-yet-lovable leader and Clark as the perennial straight man, and you've probably got a newsroom drama that could sustain itself without Superman (in fact, go watch "Lou Grant" and see if it brings up any memories.)
But all this hangs on the character of Superman, and it's no surprise that he's the biggest story-generator of them all. A modern-day Renaissance man, he's simultaneously a crusader for justice, a dedicated reporter, a scientist, an explorer, and a humanitarian. Any one of these aspects allows a writer to put Superman into a story without it feeling contrived, and he's all of them put together. Superman can start off a story experimenting on radiation, seeking out a lost city, sniffing out crooked builders, or just entertaining orphans, and it never seems unnatural or out of character. He's very much a "go anywhere, do anything" story-generator.
Which may explain why the Silver Age never had much of a rogue's gallery for Superman. There are a few memorable villains--Bizarro makes his debut in Volume 2 of the "Showcase", and Brainiac and Luthor both pop up now and again...but for the most part, Superman doesn't need more than a token villain to spend time fighting. And any of these token villains can make use of Kryptonite--this all-purpose story complicator makes so many appearances in the two Showcase volumes as to stagger the imagination. Available in green, red and blue (so far), it's always handy for when you need to keep Superman's great powers from ending your story early. Also, it's worth noting that almost all of the villains he does have possess a lot of potential for "re-use"; Mr. Mxyzptlk can pop up any number of times from the Fifth Dimension, each time with a new and unusual prank to play on the Man of Steel, and there's always a story involved.
In short, Superman showcases exactly how an open-ended series should work; his character, backstory, setting, and supporting cast (both good and evil) all function to generate more story ideas for a writer who needs them. Superman can easily sustain another 80 years of continuous publication, and though his mythos continually refreshes itself for new generations, there's no question that the ideas can go on forever.