I'm a fan of both Marvel and DC, but I honestly believe that Marvel's most popular characters are more accessible and relatable to audiences.
I don't know. I mean, yeah, I know where that argument comes from, Marvel's characters tend to be a bit more down-to-earth. But if you look at each case, I don't think the difference is that great.
Is Thor innately more relatable than Green Lantern? I don't think so. Prior to the Thor movie actually working, most people would have said/were saying that this was one of the most difficult characters to adapt.
Captain America is basically a paragon/archetype, with the historical ties and the man-out-of-time element. I don't think that's inherently more interesting than Superman, who is a paragon/archetype with alien ancestry.
Movie-verse Iron Man takes the millionnaire/playbloy/tech genius elements from the comics, but the banter, which everyone now associates with the character, was created by/for RDJ.
So, I don't think there are insurmountable problems here, for the DC characters. Somebody could make some serious cash with Wonder Woman, for example, I think. It's just a matter of getting it right.