Heimdall in the comics/movies is LOOSELY based on Norse Mythology. So loose in fact, that he HAS been drawn as a Black character in the comics.
Not a Marvel follower in the least, so that's news to me. If more people were aware of that, it'd probably calm some of the flames.
Furthermore, what Marvel has shown time and again, is that the movies are set in their own universe, a universe where it is perfectly plausible for Heimdall to be black.
We can say that the Marvel film universe is simply a different beast altogether, but at what point do we stop? Perhaps Justin Timberlake could play Thor and maybe the Hulk could be played by RuPaul (though the Hulk form would need an afro and be colored a fabulous
purple). Maybe DC could get on board, and replace Christian Bale with Paula Abdul. Teehee. :) (If I was a scandalously rich film executive, I'd totally fund that project.)
I think the casting decision simply came down to what people (the greatest number of them possible and MAYBE please some fans at the same time) can believe on screen. Personally, it kinda bothers me. Maybe I'm what you would call a purist. Or a racist. But the majority will probably believe it and make Marvel a truckload of cash.
Something the actor said did kinda irk me:
"There has been a big debate about it: can a black man play a Nordic character?" he told TV Times. "Hang about, Thor's mythical, right? Thor has a hammer that flies to him when he clicks his fingers. That's OK, but the colour of my skin is wrong? "I was cast in Thor and I'm cast as a Nordic god," he said. "If you know anything about the Nords, they don't look like me but there you go. I think that's a sign of the times for the future. I think we will see multi-level casting. I think we will see that, and I think that's good."
In fantasy, the story is carried by magic used in logical ways. No one is claiming (even the butthole boycotters) that this type of magic exists in the real world. However, there is a claim that in *that* universe, there are certain things like hammers that give you powers because that universe's gods
empowered them to do so. Completely fictional, but believable in that world. What gets tricky is when there's no explanation given (or even possible to give) for certain elements in the story. This exists even outside of fantasy and we call them plot holes. We look down upon them. Unless we're a bunch of postmodern son of a guns. Then we like it because it's "subversive."
Now it is certainly possible to play with the formula. One of the most common tropes is to use British accents even when the characters are not British. The fact it is consistently used across all the actors means that in that universe, people speak that way. There may be some characters that don't speak in British accents to show they're different in some way. I haven't seen the Thor trailer, but I can imagine they probably use British accents to make them seem kingly and refined even though they're Nordic. They may even have mortal characters in the movie and, if so, they probably don't speak in British accents as to show they are normal and mundane.
Of course I haven't seen the movie (and none of the butthole protestors have) but the argument against is built on the idea that there will be no explanation for why Heimdall is black when all his peers are white. I will admit this is simply built on what a certain audience notices first and not later or never. Upon closer inspection, we'll see that Anthony Hopkins and Chris Hemsworth are supposed to be playing father and son and they don't look as such. The possible plot hole never gets brought up... because it's not as apparent as the case of Heimdall. The people on the "against" camp simply find it as such. Those that don't care don't notice it as sharply. Little kids are like this in that they might not recognize race at all until a certain age.
At least that's how I'm understanding the issue. :)