Other than being a reoccuring series of cross over battles, Marvel Vs DC could also be seen as the pivotal question of mainstream comics. DC and Marvel have been providing, for 60 odd and 40 odd years respectively, a series of good people in garish costumes fighting with a series of bad people in garish costumes. Although both DC and Marvel have hit some artistic high points over the years, their basic fare is still super powered slug fests.
Yet comic book fans still feel that the two companies have very different approaches, and prefer one over the other. When I was growing up, I was a Marvel fan, and would have viewed reading a DC comic somewhat like wearing my shoes on the opposite feet. It just wouldn't fit. Thanks to the power of the Multnomah County Public Library, I came to appreciate DC comics. Yet I still would say there is a large difference between them.
The concentional history is this: DC comics was churning out an endless succession of square jawed heroes with gadgets and no personality, when Marvel arrived on the scene and shook things up with super-heroes who had problems and rough edges. This theory is very true, as far as it goes, Marvel's early titles did indeed far outdo the DC comics of the time in realism. However, it should also be pointed out that the lameness of the Silver Age DC comics was due to the restrictions of the Comic Code, and that as soon as Marvel started shaking things up a little, along with the general relaxation of discourse in the 1960s, DC comics soon caught up with their competitors in terms of realism. Later on, it would be DC series such as The Watchmen and The Dark Night Returns that would take the artistic torch from Marvel, and especially the artistic milestone that was the Sandman series (of which I will speak more of later).
One of the first things I thought of to try to distinguish the two was that Marvel's cosmology seemed to always be more scientific while DCs tended towards a religious or magical bent. However, this seemed to be more a function of the times than anything else. The Golden Age of DC had many magically oriented heroes, but during the early Silver Age, the faith in progress and science made Marvel and DC titles more based around science. As the nation entered the late 60s and into the 70s, magic and the occult reasserted themselves in comics.
There has been a number of other factors that have wavered within the two companies, for example the choice of having super heroes fight "real" criminals planning diamond heists (and later drug shipments) versus superheroes involved in magical cosmological adventures where they fought aliens and gods. Once again, this was often more a difference between different eras than it is a difference between the different companies.
I was at a loss until the obvious came to me, while reading a collection of Jack Kirby's Jimmy Olsen tales, where he introduced Darkseid. I was also thinking of what it means that DC has cities that don't exist. And I also, of course, thought of Neil Gaiman's Sandman, the series that showed me the artistic structure of the DC universe.
I've never visited London in my life, but for some reason, I go there at least every few weeks in my dreams, to walk around a weird version of London. I've been to New York a half dozen times, but in my dreams I am often wandering around a semi-Manhattan. It occured to me that Metropolis and the others were types of dream cities, cities that followed their own dream logic. And from there, as I read Jack Kirby's fanciful work about an underground city stocked with the clones of The Newsboy Legion, it occured to me that the entire DC universe runs on a type of dream logic, and that Neil Gaiman's Sandman was not merely tacked on to the DC universe, but is the epitome of it.
As I said earlier, Marvel and DC have 100 years of history between them, with each company putting out a dozen titles, at least, a dozen times a year, meaning that there has to be tens of thousands of issues to read. It is hard to sum up what the difference would be, but I think my view that DC comics follow a dream logic while Marvel comics at least attempt to be about "our world" is as good a guess as any. The issue of time travel is an interesting one to look at: Marvel's time travel is always fraught with paradox and logical problems. DC's time travel, from the days of the Legion of Super-Heroes up until the JSA's Hourman and the JLA's Rock of Ages storyline, has always been extremly non-linear, with time feeding upon itself in a cyclic fashion, much as time does in dreams.
I do not know if this explanation makes any sense to anyone but myself, but I think it may provide a good starting point.