The Marvel covers did not start out all that different from the DC covers, although that soon changed. The early Fantastic Four covers featured the characters demonstrating their various powers:
The characters are so iconic to us these days that it can be hard to remember that back in 1961, kids had no idea who the FF were, or what they could do, so it was important to give them an idea quickly. As I have mentioned in the past, this may be the real reason the early FF issues started out with them squabbling; so that they could demonstrate their powers early in the comic. It's also why the early X-Men stories opened with them in the Danger Room.
Spiderman's powers were a little less interesting visually, and so the early covers often featured the villain's unique abilities:
This is very much like the situation with the Flash, where the rogue's gallery became the focus of the covers. And, of course, Daredevil:
At first, the Iron Man covers in Tales of Suspense featured his powers:
But that pretty quickly changed and the covers became another "villain of the month" gallery.
A similar thing happened with the early Thor issues; at first there seemed almost an "isn't he gorgeous" aspect to them:
There are two very big differences between the Marvel covers and those of DC. First, the Marvel covers were much less likely to have speech or thought balloons on them; even as early as FF #2, Stan was content to let the picture tell the story. In fact, there is not a single speech balloon on a Daredevil cover until #34; the next one appears on DD #57.
However, they had oodles of the Lee hyperbole. The other difference is that the Marvel covers often featured the hero beaten:
Although the DC covers often featured the hero in a death-trap, implying they had been previously beaten, they seldom featured him prone and apparently lifeless. One of the few exceptions I can think of is this cover:
Actually there are a couple other Flash covers like that, but they came towards the end of the 1960s.
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