(or "Perils Of The Shared Universe!")
I've talked before in this series of columns on how linking different fictional characters in a shared universe is a boon to a writer looking for stories; since a "storytelling engine" is, after all, an open-ended status quo designed to generate story ideas for writers, the macroscopic storytelling engine of the fictional universe contains, by definition, all of the smaller storytelling engines for each character and can use all of them. (Which is a fancy way of saying, "If the Batman writer is stuck for a Batman story this month, he can use an old Flash villain to keep the audience entertained.") But is it really an unalloyed positive to set your stories in a shared universe? Does it give you all sorts of extra options for telling stories, with no drawbacks. Oh, you wish. Because as much as anything, stories are all about complications. A story that has no complications is resolved easily, and a story that's resolved easily is...well, short. Very short. Writing is the art of making the protagonist's life difficult.
Like Adam Strange, for example, the archaeologist who discovered that the planet Rann in the solar system of Alpha Centauri has been trying to communicate with Earth for years now, by beaming "zeta-beams" at us. When Adam stands in one of these zeta-beams, he's saturated with weird radiation that pulls him instantly through space to Rann. Which turns out to be lucky for the planet Rann, because they're just clawing their way back to civilization after a planetary war, and they're constantly having to deal with weird alien menaces. So Adam Strange pops in every time he can find a convenient zeta-beam to hitch a ride on, fights menaces, romances gorgeous alien babe Alanna, but when the zeta-beam effect wears off, he pops back into existence on Earth. (Like every other instance of teleportation in science-fiction, the fact that planets and solar systems and galaxies all move is conveniently ignored.) It's a good storytelling engine, particularly because Rann is such a well-conceived world; it's a mix of different city-states, some in uneasy alliances, some in open warfare, and almost all with at least a little "forgotten technology" from before the war. There's always something for Adam Strange to do on Rann.
Then, since the Silver Age had officially gotten under way, and Adam Strange was just one of many popular DC characters like the Flash, the Atom, Hawkman, and Green Lantern, editor Julius Schwartz decided that it made perfect sense to assume that Adam Strange lived on the same Earth as all the super-heroes, and that Rann was just one of the many alien worlds of the DC Universe, like Thanagar and Oa. (Not that they necessarily made this a conscious "decision", or even necessarily consciously thought of a "DC Universe". Julius Schwartz was kind of making up a lot of the rules as he went along, a freedom that many editors since have devoutly wished for.)
This, of course, led to every kid who read DC comics asking themselves the same question. "Why doesn't Adam Strange get a ride to Rann from Hawkman or Green Lantern?" After all, both of those super-heroes had space travel; Superman could fly through space at insane speeds and carry passengers (because he's Superman. Don't think too hard about it.) Adam Strange's relationship with Alanna is a bit of a false status quo, in that they both wish that there was some way they could be together more often, but it works in the early stories because you don't need to pay a lot of attention to it. Alpha Centauri is four light-years away, Adam Strange's current solution of zeta-beam hopping is the best he's going to get, and that's just all there is to it. But once you have Adam Strange meet Green Lantern, the writer has to desperately come up with some sort of plot contrivance to keep Adam Strange's storytelling engine working. In a shared universe, sometimes one storytelling engine can break another.
Over the years, they've come up with a lot of different reasons why Adam and Alanna remain "star-crossed lovers", and even decided for a time to give the two of them the happy ending they deserve by letting Adam get to Rann for keeps. This being comics, of course, it didn't last. (The current solution is simple; Adam and Alanna both prefer things the way they are. The constant travel by zeta-beams is just the modern space-couple's version of "me time.") But the fact of the matter is, they wouldn't have to worry about it if Adam Strange's continuity was just kept separate from the other comics DC publishes.
You can see this in a lot of comics if you start looking for it, from Spider-Man ("Why doesn't Spider-Man get one of the twenty-seven mutant healers running around in 'X-Men' to heal Aunt May?") to the Avengers ("Professor X's telepathy can't detect Skrulls. Really.") to Batman ("Only Batman can stop Deacon Blackfire's army of homeless people, because, um...the Justice League are all in space that week. Yeah.") A sufficiently large shared universe contains the solutions to any problem a writer can come up with...and for a writer, that's the biggest problem of all.