That reminds me. I love Superman and Spider-Man, but one flaw in that story - the only flaw for me - is the awe in which civilians seem to have for Supes' flying ability. New Yorkers should see flying heroes as ordinary, what with Vision, Thor, Human Torch, Iron Man, etc. A flying hero should be routine to their eyes.

Despite my preference for shared Earth - and why I feel DC and Marvel should - I accept it's an impossibility in some cases. When Batman meets Judge Dredd, it has to be inter-dimensional because the Dredd history of the United States is different from the DC history. It wouldn't work, a unique set of circumstances led us to Dredd's world of law and order. So with something like that, I get it.

Not sure why the Ghostbusters and TMNT need to be in separate dimensions, though. They both encounter strange beings, why can't they live in the same New York?

This is something I would have liked to have seen in the crossovers - just as the two universes presumably share a lot of the same people (ie. both had a Lincoln, an Elvis, a Caesar, etc) wouldn't the Marvel Universe also have, say, a Bruce Wayne, Barry Allen, Ma and Pa Kent, and so forth?  Of course, with no Batman or Flash showing up at Avengers meetings, one might presume that the lives of those characters followed a different trajectory from that in the DC Universe.  It might have been interesting to see Bruce Wayne, for instance, as a genuine shiftless playboy calling in Iron Man's help to help get back his mother's jewels or something which he recklessly gambled away or whatever.  Not an act - that's just how he is in this world where his parents didn't die.

Also; the idea that the two universes are actually shared, but we've just never been made privy to all those Wonder Woman/Captain America or Superman/Sub-Mariner team-ups, reminds me of a Mr. Show (I think it was Mr. Show) sketch where a newscaster reports on the death of the fifth member of The Beatles Bob Lemmington (or something) who appeared everywhere the group went but was always just out of camera range when the group's photo was taken and just out of audio range when their music was recorded.  But he was just as important as the others. 

Or that there are people with too much time on their hands

I've had this debate over time.

I know, in theory, the likes of Superman, Flash and Green Lantern could have helped Batman any number of times when Gotham City was descending into chaos. But if they were helping him every issue, then Bats would be made redundant.

Same with Marvel. Yes, when Green Goblin has left New York in flames, and Spidey is at his worst, one might wonder just where the hell the Avengers are. But in a Spidey comic, we want to see Spidey overcome the odds. If Iron Man or Thor popped by every time Goblin was on the rampage, Spider-Man may as well not have his own book, eh?