Since Hollywood officially ran out of ideas ten or fifteen years ago we've been treated to an avalanche of sequels, remakes, and adaptations of source material ranging from video games to classic works of fiction to comics. When you're remaking everything all the time you have to deal with the fact that the audience's expectations are affected by the earlier versions. I mean, after you've made 10 different Batman movies the next actor to take the role is limited by how the previous actors played it.
Sometimes casting decisions complicate things when a role is handed off from one actor to another. Some casting decisions are baffling, like choosing an overgrown fratboy like Vince Vaughn to reprise Anthony Perkins' role in Psycho. In other cases the problem is that an actor's appearance does not match the viewers' image of the character. Like, for example, when the upcoming remake of beloved children's movie-musical Annie casts a black girl in the title role. Being a previous Oscar nominee, I'm fairly certain that said actress can handle the not-terribly-complex film.
Admittedly, the fictional character of Annie is pretty closely associated with the "red hair and freckles" image. It might be odd to see someone else do it. But honestly, is it that big of a deal? After all, they call the profession "acting" for a reason – it implies (stay with me here) "acting" like you're something you're not! An actress isn't Annie any more than she is Queen Elizabeth or a serial killer or a talking pig. Why I've even heard that sometimes all-male or all-female schools manage to put on plays where the people who fill some of the roles aren't even the right gender. Because it's acting, and you suspend your disbelief as an audience member. Since the role in question in Annie is a fictional character, I hardly see the problem.
Were it a biopic, the actor's race might be an issue. You wouldn't cast Wesley Snipes as Pope John Paul II or Ed Asner as Malcolm X. When we're talking about a character that doesn't exist except as a figment of our imaginations, is it really that hard to suspend disbelief to think that, for example, Idris Elba is a mythological Norse god? If you're stuck on that question, bear in mind that Norse gods are made up and never existed so no one has any idea what they look like.
Oh wait. I keep forgetting. Black. The problem is black, even though we're totally not racist.