Vulture looks at the trend toward casting dirty forners in super-hero movies:

The ugly truth is that American leading men just aren't terribly manly anymore, says John Papsidera, the casting director on both The Dark Knight and The Dark Knight Rises. 

"You look at the list of American leading men, and in their twenties and thirties, they're very boylike," he says, adding, "Take Jesse Eisenberg: I put him in Zombieland, but he's not going to play Superman. He's much closer to what Dustin Hoffman turned into than John Wayne or Steve McQueen. It's hard to find movie stars that live up to the needs of the story. Leo [DiCaprio] is growing into it, but for a long time, he seemed young and boylike. Inception was the first time Leo seemed to have fully grown into a man. You need to find guys who carry that heroic-ness with them."

Later an agent notes that the big American actors tend to avoid comic book movies. Robert Downey Jr. seems the obvious exception. There's also the fact that, in the main, black actors are generally shut out of any leading role. But Black Panther's coming.