In April 2009 alone, DC will print twenty new issues or volumes based on Superman. That counts his two "solo" titles (which happen not to feature him at all lately), a number of team titles (not counting JLA, which had featured him until recently), collected editions of Superman stories from over the years, and single issues devoted to characters who were strictly derived from Superman (eg, Supergirl). That's not counting Captain Marvel (who, a court agreed, copied Superman in concept) or for that matter, all of the superheroes published by all comic book companies who were inspired by Superman. Twenty. And that's not an atypical month.
In the Fifties, Superman regularly appeared in four stories a month (three per issue in Superman, plus one in Action Comics) plus World's Finest. During the Bronze Age, Superman appeared monthly in Superman, Action, World's Finest, DC Comics Presents, Justice League of America, Super Friends, and Superman Family. Overall, Superman has appeared in an average of perhaps 50 comics a year over seven decades, in addition to representations in other media like animated series. As a rough estimate, we can speculate that there have been about 4000 Superman stories. Unless you count individual episodes of a soap opera as a story (they tend to feature each character for only a small portion of each episode) or a daily newspaper strip (same proviso), it's hard to name another character who has appeared in that number of stories. Well, besides Batman. Much of what I say here applies to Batman as well.
That's a lot of space to thrill and a lot of space to fill. How does one tell 4000 stories about one character? In a nutshell, one covers a heck of a lot of ground, one repeats a lot of story ideas, and one isn't "one" at all because this is a character who has been passed around to hundreds of writers since the original creators have long since moved on (and passed on).
There is a framework of three kinds of story that has filled Superman's history:
1) The Legend: The absolute immutable features of the character. Father sent his son to escape a dying planet. Arriving on Earth, the son was raising by good farm people. When he was old enough, he used his amazing powers to fight for justice. Held down a day job as Clark Kent. While a few variant tellings have shaken up even these axioms (Stan Lee's version comes to mind), these basics have not been changed often. There's also not a lot of material here -- it's not much longer than my four-sentence summary.
2) The Adventure(s). The basic Superman adventure. Maybe the villains are from organized crime, or maybe they're from another galaxy. Maybe they fire pistols from their getaway car, or maybe they bend space and time, but whatever their plan is, it's ruined when Superman showsup. This story has been done a thousand times, but it's really one basic plot. It's therefore, by and large, fairly repetitive. The names and faces of the villains change, but the plot has a predictable form. It's because these thousands of stories are so similar that I put the "s" of "Adventures" in parentheses. To some extent, it's the same Adventure over and over again.
3) The Mythology: While most Superman stories change nothing about his life in any enduring way, every now and then something significant would happen and add a meaningful chapter to a slowly-growing mythology. Usually, these changes, once rendered, had a life of years before being revisited.
For the most part, (1) is a single story. It's been told many times with (usually) minor tweaks, but can't be changed greatly without it being a new character (like Stan Lee's "Superman" or Captain Marvel).
The majority of Superman stories are in class (2). If you look at a typical decade of Superman stories, you'll see a large number of (2) stories with a small number of (3) stories. For example, once Kandor appeared as a bottle city, 21 years passed before Superman succeeded in restoring it to full size. The introduction of Kandor in Action Comics #242 was a major event in Superman's life, shaping many stories over the years to come. But it was atypical: The very next story, for example, involved Circe turning Superman into a lion-man and his finding the cure to turn himself back into a man by the end of the story. When Supergirl was introduced in Action #252 (a major event), the next story in Action was about a Kandorian criminal escaping to vex Superman before being defeated. The overall pattern has been pretty stable over the years. Major stories have to remain rare because Superman's life story is not a story of 4000 consequential events. Although a humdrum story by Superman's standards is usually a bigger adventure than anyone real ever experiences. (When's the last time you fought an escaped alien?)
Because (3) stories were rare, he accumulation of mythology from 1938 to 1986 was gradual. In the same way that one year for a dog is like seven years for a human, you could say that seven Superman years was about one person-year. When Superman had been in 49 years of comics, he'd perhaps "aged" seven years from his debut. And by "aged", I don't mean that he looked older, but that he'd accumulated enough personal history to correspond to about that much time. A great summing-up of the first 40 years of Superman was in the Great Superman Book, which listed, encyclopedia-style, all of the big players in Superman's life and many of the smaller ones, although it conspicuously ignored any mention of the Earth Two / Earth One distinction that DC had been using for several years.
The repetitive nature of type (2) stories forced some accomodation. I think a useful perspective is in Margaret Boden's analysis of creativity in general, The Creative Mind.
She (with no reference to comic books) observes that in any creative medium, you see the various artists in that field explore a certain "space" of what is acceptable, eventually covering so many of the possibilities that new work starts to seem to repeat what has already come before. Eventually, there is a break-out: Someone does something so new that it brings up some truly new possibilities. Of course, some of the audience loathes the new work (imagine playing gangster rap, or ragtime for that matter, for people who only like Baroque music). In time, the new "space" is also exhausted and a new revolution has to happen.
Superman's stories have followed the same path. At first, he only faced realistic threats, such as corrupt politicians and businessmen. Eventually, he faced mad scientists, and then all kinds of wild science fiction threats. His personal life began to become a meaningful topic. And finally, when the accumulation of mythology seemed too cumbersome, and a break-out was needed for new kinds of stories, DC gave John Byrne the reins to utterly revolutionize Superman. He kept (1) the same (basically), and told plenty of (2) stories, but with a new feel to them, and a totally blank slate as far as (3) went. Given that blank slate, Byrne and his successors wrote a comparatively high percentage of (3) stories, refilling the blank slate they'd been given. (3) stories are hot -- they're exciting and they're (potentially) less repetitive than (2) stories. The post-Byrne writers not only filled Superman's mythology much faster than the pre-Byrne writers, they also stepped all over each other's toes, retelling Superman's origin, and creating multiple redundant versions of some supporting characters. For each individual writer, (3) stories were a huge enticement. The "gold rush" to redefine Superman led to excess, and a greater (and sooner) need for cleaning house after 20 years than was needed before after 49.
One soft reboot later (Infinite Crisis), Superman is receiving a new mythology thanks to Geoff Johns and other writers. It's attractive, well-thought-out. It takes the best of various versions from the past (Steve Lombard and Cat Grant, together at last). It's perhaps just exactly the way Superman should have been done in the first place.
Nevertheless, it's a perilous path to walk, and so far, lacks thrills. The problem is that retelling old stories, tweaking the details -- even when improving them -- is still retelling old stories. In Action #242, when Superman entered the bottle city of Kandor for the first time, it was a dramatic moment. We'd never seen more than three Kryptonians at a time since the planet had blown up. This was a curveball the reader couldn't possibly expect. But when Superman "discovers" Kandor in 2008, the shock is his alone. Almost every reader knew that Kandor was going to be found. This isn't a matter of shock or wonder any more than it is funny to hear a joke the second time. Worse, this is not only the second "introduction" of Kandor, not even the third, since a post-Byrne Kandor had already been introduced and re-cast. It is the fourth or fifth version of Kandor.
Johns (et al) inherited a mess. Re-inventing the mess might possibly create a clean Superman mythology that future writers may profit from. But for the time being it is uninspiring storytelling. The mythology was exciting the first time around because it was the first time around. When asked what he thought of kids imitating his band by wearing Beatles wigs, John Lennon responded, "They're not imitating us because we don't wear Beatles wigs." I approve wholeheartedly of the particular details in the new Superman encyclopedia that Johns is writing for us, but it's neither improving upon nor matching the Silver Age Superman because he wasn't a bullet-point-by-bullet-point retelling of a previous version.