That ten year gap was necessary to allow the general readership to turn over a couple of times and make the Silver Age possible. It was also just a short enough span of time that a few of the Golden Age readers were still around and interested. 'Zines either sprung up or were revived in response to the new crowd of characters, and the discussion was actually very civil and almost universally in favor of giving these new guys a chance... So long as the older versions got some time on-panel eventually as well, of course... "You DO intend to have Alan and "Hal" meet at some point, don't you?" 

Having written a great many of those Golden Age stories featuring the JSA themselves, Gardner Fox, Rob't Kanigher, and Julius Schwartz were happy to accommodate the nostalgia. 

Also, being professionals in the field of sci-fi magazines and paperbacks, the explanation that the Golden Age adventures of one world had come to the writers of another in dreams wasn't going to any great lengths. It was simply thinking the concept through and doing so in an imaginative manner.

This was the same creative team after all that gave us Captain Cold's cold gun being able to produce realistic-looking illusions because if extreme heat can produce mirages, well, then extreme cold would be capable of doing so as well... Is that based on scientific fact or actual examples? No. But it is the result of trying to imagine just what a gun that produces absolute cold might be capable of in the most imaginative circumstances.

It's interesting that the initial crossover between Barry and Jay both allowed for Jay to replicate the trip between Earths in the future should he wish to do so, allowing for another story featuring the two, but it also kept the parallels confined to just the two Flashes and a collection of villains from the Golden Age. The JSA and the question of Clark Kent, Bruce Wayne, and Diana Prince doppelgangers are not addressed. There was no reason to open a can of worms if they decided NOT to revisit Earth-2, after all.

The very next crossover between Flashes did address that directly by allowing the Earth-2 Wonder Woman to participate, as well as select members of the JSA. Even then, it would be a few years before the E-2 Superman and Batman showed up, and those tales were done by fan-turned-pro Denny O'Neil in JLA #73 and #82, respectively. 

Since there had been no break in either character's publishing history, superficial differences between the counterparts had to be contrived to separate them from their Earth-1 selves. (The Daily Star and Batman's yellow oval, for instance)  WW's history was more chaotic, allowing for specific breaks at various points when Kanigher changed direction on the book i.e. his new origin for the character or the introduction of the Wonder Family. Even so, no one set about drawing any hard lines in the sand for quite a while. That was left for the fans to debate. I do think its amusing and canny of Fox, the originator of the idea, to have avoided the question altogether. Schwartz, the other fellow responsible for Earth-2, was clearly not going to stand in his writers' way if they thought they had the answers, and imperfect as those answers were, I think the construct held up well enough that it could have gone on.

The number of times that fans-turned-pros have tried to re-instate parallel worlds or some semblance of them speaks to the impact the concept had upon DC readers over the years, despite heckling from the stands from Marvelites and anal retentives shrieking about various Aquamen, Wildcats, and Spectres. (Oh, my!)

Really, it was fine. Better than that, in fact. Earth-2 was one of my favorite hang-outs as a kid. I liked that place and those people. The heroes there had long since earned their stripes and were deserving of the respect accorded them. The idea that the stories could have greater impact on the lives of the characters was important as well. There were elements of grace and charm as well as honesty to their careers. They really HAD been there for the good times and the bad. Heroes could die; lose their fortunes; have children... The generations could be at odds. The Earth-2 heroes could lead lives that were more representative of real life, since they were not bound as tightly as their Earth-1 counterparts to the need to pop back up again next month, same as before, in order to sell this month's batch of comics. 

Yes, time was outpacing the conceit that WWII heroes could still be vital and strong, but there were ways around that (Nick Fury and the Infinity Formula, anyone?*) The way it all came undone was deeply disrespectful and disappointing. And we got nothing in return for all the demolition, confusion, and ineptitude that we couldn't have had otherwise. 

* The JSA was reinvigorated at least twice, once when Ian Karkull exploded, and again when they returned from their post-Crisis exile in Roy Thomas' obsessive-compulsive wonderland, the Nibelung Saga. (See also: the FF and the Skrull anti-aging ray.) The idea that they were all "just too damn old and needed to die" doesn't wash. Like early Marvel, the JSA's adventures on Earth-2 initially took place in real time, but they did not have to continue doing so.

Showcase #4 is the one that started the ball rolling and created the model the rest of the line followed. It introduced a new, scientifically-based version of a pre-existing character with a completely new identity and setting, as would be done soon after with Green Lantern, the Atom, and to a lesser degree, Hawkman.

Detective Comics #225 introduced J'onn J'onzz, the Martian Manhunter who would later go on to become a super-hero in the pages of the Justice League and House of Mystery. I maintain that at the time of his introduction and for years afterwards, however, he was a "gimmick detective," not far removed from Roy Raymond, TV Detective and Pow-Wow Smith, Indian Lawman. His shtick was that he had alien powers, new ones each issue it seemed, but the stories and tone were consistent with other, non-super-heroic, characters of that same era. His introduction two years prior to Barry Allen did not kick off the start of anything.

Julius Schwartz, I suspect playing the gadfly, has asked why Captain Comet, a generic DC spaceman with mental powers due his birth as a mutant, isn't considered the start of the Silver Age. He was also brought into DC's super-heroic pantheon at a later date and became a mainstay of the Secret Society of Super-Villains comic in the 70's. If J'onn J'onzz counts, why not Captain Comet? That would put the debut of the Silver Age as far back as 1951. 

Also worth considering for fans of the Martian Manhunter as DC's first Silver Age hero is the presence of Roh Kar in Batman #78 from 1953. Roh Kar is also a green-skinned alien policeman who works with a known super-hero and precedes J'onn by two years. Why isn't Roh Kar the dawn of the Silver Age?

I reply that it's because he didn't lead to anything else, in much the same way that Captain Comet and the Martian Manhunter didn't. It was Barry who broke out of the doldrums of the 50's super-heroic malaise and pointed the way to future. The sales on his book brought back interest in publishing costumed crime-fighters and led to the JLA, which arguably led to Marvel Comics. 

Without Barry, none of the other dominoes fall. J'onn precedes him, but without the JLA, filled with Barry-like Silver Age recreations of once-popular heroes, J'onn languishes in obscurity alongside Mysto, Magician Detective, and Captain Compass, the Sea Sleuth.

It wasn't necessary for this to be referenced by other Marvel characters outside the Transformers series .  But the argument that we don't see every moment of a character's life and that is an explanation for why certain major things won't ever be mentioned "on-screen" doesn't cut it with fiction.  It makes sense in real life that we don't hear about everything because we don't see everyone all the time, but in fiction, dialogue and scenes are set up for dramatic effect, not to be completely realistic.  Readers assume they'll see all the significant moments or interesting details even if they're not absolutely needed for the plot.  You could say that only matters within a story, not for details from another series.  But this carries over into fictional shared worlds even though references to separate series aren't needed to understand each story.

Some readers naturally expect in fiction, especially a large, shared fictional world, that dramatic points are always mentioned "on-screen",  including when you want to get across the interrelated nature of it from time to time.  It's not that you have to mention every single thing that's happening in every other Marvel series or that the main character has to reference every event from their past all the time or how many times they've fought a certain villain, but a few details mentioned here and there, just enough to continue that illusion of a giant, shared world, is a way to handle that dramatically since this is fiction, not mundane real life where you don't see and hear all the significant details.  It's just not satisfying in a dramatic, fictional context to say, well, just imagine it happened "off-screen."  It's more exciting to see it.  You're reading these series to see a depiction.  It was great to actually see S.H.I.E.L.D.'s reaction to a new artificial race battling over resources because it's exactly what they deal with, global security concerns.  I guess you could argue that shared worlds train readers to think this way to the detriment of that world.

A race of giant robots on Earth like the Transformers would be big news around the world in this fictional setting.  It didn't have to be mentioned in any other Marvel series, but I can see the scale of the Transformers' two factions being a good reason why fans would get enjoyment from seeing that.  I think this is similar to John Byrne's example of fans asking why didn't the Avengers show up when the Fantastic Four first encountered Galactus.  It was leading to fan obsessiveness with putting too much interrelation into more and more stories in a way that detracts from just focusing on each story.  I can see that as just a genre convention for Marvel and DC, that you don't have to explain where everybody else is when something big happens for the sake of not having those details intrude on a story.  I'm just saying that I don't think using the reason that we don't see every character's thoughts and conversations about dramatic events is a good answer for this with fiction.  Of course in fiction some information can be withheld when it's going to be revealed at a certain time.

It depends on what it is, because it would also be natural that certain things aren't mentioned if it's going to come up later.  I think John Byrne wanted to show that two of the X-Men had met in Hidden Years or that they had met before their first appearances but when he asked on a forum about it, fans were criticizing the idea that these characters had never mentioned meeting before.  That was unreasonable.  If only this could be applied at the right time when it makes sense and would be fun to see, without fans getting obsessed that no new little details can be added without everything needing to have already been mentioned.  Even in fiction some dramatic details can be off-screen if it's going to eventually be acknowledged, like revealing that certain characters had known each other before their first on-screen meeting.  It's also something small scale enough that I can see them just not talking about it until later.