I'm not massively into comics, even less so the American mainstream, but this occurred to me during a conversation with students: you can draw a line between the two camps, broadly along the lines of their identity. DC heroes are heroes first and normal people second.
Take Batman; he doesn't have super powers, but mentally he is batman, with Bruce Wayne being a persona he has to put on to hide who he really is. Superman; he's really Kal-El from the planet krypton. Clark Kent is just a cover story. Wonder Woman, again, magical bdsm demigod princess first, nurse second. Diana Prince is a cover story; she really is Wonder Woman.
You could probably argue that supes is a bit more complex, because he was adopted and might border on a split personality; I would still argue that he is Kal-El first, Clark Kent second. He has to filter out his powers in daily life.
So what about marvel? For comparison, they seem to be normal people who have their superhuman abilities thrust upon them, and while this still makes them a power fantasy, it's a more accessible one for teenagers. Spider-Man is a good example of this; he is Peter Parker, trying to make the best use of the gift/curse he has been given, and fit into the role of a superhero. Captain America, the hulk, they were born as norms and gained powers in the lab as adults. All the X men and the fantastic four feel like people with powers, rather than superheroes. Unlike the comparable batman, Iron Man is a bloke in a suit, not an identity (despite what the unconvincing ending of IM3), and the costume is an extension of iron man's billionaire maverick playboy persona.
Thor is obviously an exception to this, but it's the exception that proves the rule* . On the dc side, green lantern might be an exception, but I don't know anything about him. Grant morrisson's run on animal man is definitely an exception to the style of dc comics, but then I don't think you can count anything that GM does as normal - it was so wierd to have that character arc precisely because dc heroes don't have the crisis of self-confidence that animal man goes through. I think it would have been much less unusual to do that to a marvel character.
I think this is a reason I've always felt drawn to marvel, their heroes always felt cooler, more youthful to me, but now i realise both appeal to different parts of the teen psyche; DC heroes are lofty aliens, who must try to fit in, like social-angsty teenagers from space; marvel heroes gain new abilities and must learn to control them and use them for good, a clear metaphor for teenage metamorphosis.
So, am I on to something? Or am I hopelessly extrapolating from inadequate experience?
*this is not true. This phrase 'the exception proves the rule' is complete balls, and anyone who uses it unironically should be given a mandatory course in logic and grammar.