I'm kinda late to this, but really, it's not about racism. People (most of them) aren't saying this silly or stupid because they're horrified by a filthy negro brute (emphasis on exagaration here) portraying what is supposed to be a GLORIOUS MAN OF THE WHITE MASTER RACE. Nah. The problem for many here is that it's just a really weird thing to do, and kind of sucker-punches the source material. Was there any real reason for him to be black in this? Did it somehow really change anything, or was there any emphasis on it? Because if not, then it just seems like a pointless thing to do.
(On a sidenote, I hear he's really awesome in the role, which certainly evens it out a bit.)
And yes, I know there's an asian actor also, which is the same deal really.
It's kind of like casting, say, Aragorn as a black man. Or making Conan an asian. Or, I dunno, portraying Wolverine as an aborigine. Or making a movie about feudal Japan or an African tribal society and sticking a white actor there without any explanation or reason, and portraying just another member of the society. It's just weird and confusing, and doesn't really add to anything.
So no, people are (in general) NOT bitching because they only want to gaze upon the fortright brow of the great white man and shun the filthy lesser races for so horribly and frivolously displaying themselves before their eyes. People are bitching because they feel it deviates too much from what they already know about it.
Then again, Marvel pretty much raped Norse mythology up, down and sideways by making their Asgard in the first place (which is still some people's actual religion, and as such I'd consider it pretty damn rude). Kind of like if you made a movie about Jesus as an alien from another planet who defeats the forces of the devil with sweet kung fu moves and his great big cross. Or a movie where Buddha launches hellfire and lasers from his eyes and fights against the anthropomorphic manifestation of human desires so that he can reach the magical sky castle known as Nirvana.
So maybe we could consider it a small payback.