While a large part of this is moot because both comic and film canon has the Asgardians as being aliens that were worshipped as Viking gods (a common theme in Jack Kirby's work: see the Eternals and the New Gods), I can still see why this is an issue. As much as people want to pretend that race doesn't matter, it still does. For example, if you took the same character traits that Peter Parker would have, and put them on a black teen, that does change the context of the character in subtle ways (I can imagine that the bullying Peter endured in high school would take on a slightly different dimension, at least). Similarly, if Heimdall is depicted as looking vaguely Scandinavian, that's completely different than seeing him as a black man. Any changes from the source material does give the character a new interpretation (see also: Joe Chill dying in Nolan's Batman films, whereas in the original story he got away, and in the Burton films he became the Joker. Three different interpretations that cast different lights on the same characters).
For all those saying that it's tokenism, though, I can't help but disagree, since the choice to make 'the Whitest of the Gods' black seems a bit too deliberate, as if Branaugh is suggesting that the Vikings depicted Heimdall as white on purpose, because they couldn't accept a god that looked nothing like them. It goes to that same phenomenon where all these depictions of Jesus of Nazareth as a white man, even though there is staggering amounts of evidence that he was black (like his parents choosing to hide out in a region where whites would be noticed; it even says in the Bible that Mary's lineage has ethnicities that were known to be dark-skinned).
So, I guess where I come down on this issue is: if it's being done to give a new, interesting wrinkle to the (back)story, then I am all for it. If it's just being done to score bonus points with minorities, then I think it will show in the performance and the writing.
EDIT: Anyone else realize that the setting for the movie is New Mexico, as in, the heart of American UFO culture? They are definitely playing up the 'aliens masquerading as gods' angle.
