There are plenty of skilled actors out there. What does this particular actor bring to the table that demands that HE be given in this role, given the nature of the character (to wit: a Norse god) that argues against the casting. You would not hire a black actor to play Abe Lincoln, or Robert E Lee, regardless of how skilled they were, because those people were not black.
Oh, don't be silly. We're not talking about a serious historical drama here. Hell, we're not even talking about a serious adaptation of classic Norse mythology. We're talking about THE MIGHTY THOR, which stopped being a faithful adaption of the Norse myths about the time Thor fought the "Stone Men from Saturn" in the very first issue of the comic book.
It's a comic book action movie, filmed in 2010. Who cares what race Heimdall is? He's a modern interpretation of an old Stan Lee/Jack Kirby character. And the days when all comic book characters had to be white are, thankfully, long past. Welcome to 2010.
Won't hurt the movie one bit.