7 books in The Dark Tower series.  12 books in The Vampire Chronicles.  12 books combined in Asimov's Robot and Foundation series.  85 episodes in the anime series Robotech.  12 issues of Watchmen or 44 issues of Next Men. These series can maintain more consistency because the scenario comes to an end, they don't need room to be flexible in the future.  So fan websites like the one for Robotech with a detailed chronology or something like the Dark Tower Companion or the Vampire Companion can work.  The creators are not restricted by compiling information from the series.  And other writers who continued some of these series followed the continuity since these were not going to be open-ended continuations and science fiction novels or TV series are treated differently in terms of continuity.  The Robotech series did make some revisions many years later but usually these series don't need to go back and add or alter anything later so it's not a problem when fans are engrossed in the details that can be locked down.

It becomes a problem when that type of thinking gets a hold of long running series that are made to go on indefinitely.  When genre fans see any long running series it's assumed they're all meant to work in the same way, which is establishing a fictional world that feels real because of the consistent continuity.  Because Marvel and DC have been around for so many decades, by the 70s or 80s, they fell into this trap with fans who didn't distinguish between let's say Star Wars or Lord of the Rings and continuity that's more like The Simpsons or Calvin and Hobbes.  So I don't think it's this way of being a fan that's wrong but this failure to understand how different the super hero genre is even though there are some similarities to other fantasy and science fiction properties.

Hence Johns' idea to use Dr. Manhattan's omnipotence -- and lack of empathy with human beings -- as an explanation for the rewriting of the DC multiverse's reality, making much of it as "dark" and hopeless as that of WATCHMEN. 

And Johns has said that if WATCHMEN is a critique of superhero genre conventions, DOOMSDAY CLOCK is a counter-critique, i.e., he's trying to make a point.

That certainly gives it more right to exist than BEFORE WATCHMEN, which was a quick cash grab and not much else. 

"A darkness from somewhere has infected us. It has for a long time now, I think. Even before the Flashpoint."

(The end of FLASHPOINT #5 was the start of the New 52 -- Barry Allen changed history and tries to bring it back to normal but instead a "new normal" emerges.)

This is Johns echoing Moore's point about how WATCHMEN (and DARK KNIGHT, I'd say) became bad influences on mainstream superhero comics. 

I thought these two ways of treating it would be roughly equal in appeal to readers of the superhero subgenre of science fiction.  Or at least the separate universe model wouldn't be so strongly disliked.  I can see why trying to explain every inconsistent Golden Age appearance of a character by slotting them into one world or another is too complicated and tedious, but Marvel and DC as alternate universes is a simple SF concept with only two worlds to deal with.  I liked the way the Superman and Spider-Man and Batman and the Hulk treasury editions dealt with it, but the Avengers and JLA series was dramatic in a different way.

The model that everybody lives together and simply never mentions it is deemed too simplistic for some. There are things that MUST be explained if that is the case. Where are all the fictional cities placed? The fictional nations? 

The problem with alternate universes is that different contradictions and complications ensue from that point forward. If the Universes are, in fact, gigantic, Transformer-style brothers, why has this never been mentioned during cosmic-level conferences between all of Marvel's heavy hitters? Why didn't Kismet and the Master Summoner show up at any of them? Were the residents of Transformer-robot-guy #2 not invited? Did Thanos stop too soon? Killing half of everyone in one universe isn't really all that much of an accomplishment, numerically speaking, if there are, in fact, multiple universes out there. Doesn't he have to go kill half of everyone everywhere else now? Was Michael/Korvac going to make everything perfect in the Marvel Universe just as a warm-up before moving on to the next? And the next? And the next? 

DC apparently has room for fictional cities Marvel does not because its Earth is bigger, according to Avengers/JLA. So, is the circumference of DC-Earth bigger than ours as well or is Marvel's smaller?

The parallel universe explanation only kicks the football a little further down the field. Questions pile up nonetheless. So long as it's never going to sync up anyway, I prefer the more human-scale, down-to-Earth explanation. Not everything has to operate on a Multiversal scale. We really do not need to account for every little Wildcat, Super-Son, and Terminus to accept a given story before us. 

I do agree however, that the Avengers/JLA book was its own thing and written for a certain type of reader. There is no reason not to service those customers as well. For those who want that sense of Universal inclusion, well, there it is. I would just rather that all crossovers from that point forward* not necessarily have to conform with the rules it set down.

Besides, if we fix every jot and tittle before it gets into the hands of the uber-fan, what is he going to do with his time? Someone has to chronologically measure and gauge every event and non-event, don't they? If they don't find any mistakes, where's the fun for them? 

* Not that any seem likely. 

Kurt Busiek mentioned in an interview with Comic Book Resources that his Avengers and JLA series replaced the Marvel vs DC 1990s series for the first interaction between fictional worlds.  At least with his series there were less inconsistencies, but you made a good point that they never sync up completely.

Relating this to what started the thread, since there are so many alternate universes anyway, I think at this point, every issue of a Marvel and DC superhero comic should just be its own world.  This came up in another thread or this one I think.  They're almost there anyway with so many variant universes or timelines, which was inevitable when you have to keep grabbing the same audience's attention because you have to make major changes to do that, not just minor variations every month.  It takes more than that to get the same readers coming back.  Every story could just be drastically different from the last, following no history.  At all.  Except in as a general basis like in Return of the Dark Knight or Days of Future Past or Marvel's The End stories.  Stories could go in any wild direction you see in those unrelated type of stories outside the mainstream like the Warren Ellis mini series Ruins.   Or alternative continuations like Claremont's X-Men Forever or JB's X-Men Elsewhen.  

By trying to force the continuity to go on, in a way that's continuously compelling for an audience that's hung around for so long and mainly wants continuity based stories, they end up distorting the characters or adding too much to the origin anyway, to attempt drastic changes like the recent Secret Empire story or this new series about the first Venom host.  Stories like those that feel like they might as well be alternate universe What If-style stories anyway.

Each story could be as unusual as it wants to be.  Since projects with strong deviations are what seem to grab the most attention anyway with the casual, wider audience, and with the hardcore fans.  Let all the rules go.  It's like some comments I've seen about The Simpsons.  There's nowhere left to go within the confines of that world. They've done everything they can do with the characters.  No more fresh stories within the same framework.  

This way you'd get self contained stories all the time.

Maybe if there was still a cycle of new generations reading these series it wouldn't matter.  I never completely understood why the distribution system changed so drastically to put a halt to that cycle.  On one hand, I know about Marvel's plan with having its own distribution system and companies all pulling out of the convenience stores, newsstands, drug stores, and supermarkets, but on the other hand I remember reading Steven Grant's column explaining that the price point of comics was too low to justify the space they took up compared to magazines.  Since the sales were so much higher before the direct market took over, I always wonder if that is the reason the big publishers like Image, Dark Horse, not just Marvel and DC, don't try to go back to that type of distribution.