Thankfully, most responses have been supportive of the Elba casting. That said, a lot of them still miss the point of casting him. Take this summary of the issue by Monika Bartyzel at Moviefone:
At some point we all have to realize that changing the skin color of a fictional character is not an affront to anyone, and should be seen no differently than a different hairstyle, a modernized wardrobe or any of the other changes that fall on fictional figures...
Unfortunately, it's precisely this kind of fallacious "color-blind" theorizing that allows critics of the casting-decision to turn around and accuse the author of the Moviefone critique of being a hypocrite:
Could Bartyzel be any more of a hypocrite? She thinks it’s wrong to put whites in the roles of non-whites [she criticized a decision to cast Mickey Rourke as Genghis Khan in another film] but more than acceptable to put non-whites in the roles of whites. In fact she says it’s “racist” to object to putting non-whites in the roles of whites.
Which is a fair enough response - in spite of the fact that most of the arguments on the site are racist gibberish - given how terribly Bartyzel articulated the reasons for opposing the Rourke casting but supporting the Elba decision.
But Bartyzel is ultimately right, even if she goes about explaining it the wrong way. What it actually comes down to isn't color-blindness - as if that's possible or even desirable - but representation and power. We should support decisions like casting Michael Clarke Duncan as the Kingpin or Elba as Heimdall because a) actors of color are under-represented in Hollywood film and TV, and b) the source texts of comic book adaptations are often 50 or more years old, and so were made for all-white audiences and, predictably, feature all-white casts. (And, often, these are subtly racist texts that were produced for an explicitly racist audience. Reproducing those texts exactly, just for the sake of creating a faithful adaptation, means reproducing those racist politics or representation, too. Fidelity for its own sake is often a bad idea.)
Hollywood is full of these kinds of adaptations and full of films populated by exclusively or almost exclusively white casts. What Hollywood is not full of - aside from the films created by Tyler Perry and a few other films marketed specifically for black-audiences (films that often still manage to find space for white actors, mind you) - are meaningful roles for non-white actors.
I'll simplify it, even, and say that representation is power. It's empowering to see images of heroes that look like you, that you can imagine to be you, and disempowering to feel that you either can't identify with them or actively disidentify with the people who look most like you. There are plenty of white male superheroes, wizards, demigods, and so forth, and comparatively few black men playing similar roles - virtually none once you remove any that have been played by Will Smith.
Idris Elba isn't stealing from a scarce supply of white male fantasy roles, but he is contributing to a scarce list of black male fantasy characters. And that's valuable and interesting in a way that, say, Sam Worthington as Heimdall would not be.