Elba's casting is undoubtedly going to cause a bit of a stir, as the Heimdall of the Marvel comics is a redhaired Caucasian. In the original Nordic myths, he was even called "the whitest of the gods" which seems like a thousand tasteless jokes in the making. (For the record, "whitest" refers to the light he emanated, not his skin color.) I imagine a lot of people are going to make those jokes, complain about things being PC, and just be very unpleasant.
But you know what? The Asgardians are gods. They can be any color they want, and should be. Marvel's Thor is also distinctly different than the original Norse myths, and I think a diverse cast is a fantastic and appropriate idea. Plus, this is Idris Elba. He's the kind of badass you want on that Bifrost Bridge, guarding the city gates, and having throwdowns with Thor. (He's always trying to prevent everyone's favorite blonde from going in or out of Asgard.) Kudos to Kenneth Branagh for recognizing that, and for creating an Ultimate Heimdall in the course of pre-production.