I think all of the DC heroes or many of them are quite hard to square with the reality of the world nowadays. I know in comic terms they get round that by them being set in different worlds, but really..?
Superman is an alien. Wonder Woman is like, some kind of heir to being a goddess. They don't really have the best backstory. (I know there's many more but I've never been a particular DC fan). DC comes across as quite campy, quite old fashioned, whereas Marvel seems to be able to move with the times.
I believe Marvel's main success is due to X-Men, X2 and Spiderman establishing it in the industry. These are the films people remember. They were cast and acted quite well and the themes in them overreached the typical action summer blockbuster, delving into romance, social inequality, and in X-Men, lots and lots of symbolism. Then later films like Iron Man could build on this core.
Obviously they didnt all work out. With something like Fantastic Four, the idea of the 'super-family' just seems too neat and unrealistic, there's not much you can build on there with 'cosmic rays' and 'radiation' etc.
But with DC heroes, its even hard to create a realistic story. With Spiderman, you could see why Peter Parker was bullied and outcast. Why would that happen to Clark Kent? Because he sometimes wore glasses and had to always run to the bathroom when a distaster happened?
If DC could retcon its entire universe into a united kind of 'film universe' the way Marvel has done it would work wonders for them.
At the same time though there'd be difficulties because of the scope of all the characters involved. Superman, Wonder Woman, Green Lantern, Batman...these are all really powerful characters in their own right. In something like X-Men all the characters have their own little 'niche' and situations in which they can lose are easily found because they arent all that great alone.
Having said all of that, to take this path now would perhaps seem as if they were attempting to capitalise on the success of Marvel by copying them and this might also be received poorly by critics and moviegoers.