Cracked had an article about this. Now, for the most part, I agree, this topic brings out racism like nobodies business, but...
We want the characters to look like the characters we know, right? How is that wrong? Like, yes, Johnny Storm has personality traits (not to mention his iconic power) which we can identify him through, but if he doesn't LOOK like how we picture him, its strange. That's quite apart from the idea that, however racist we, the audience, might or might not be, we can't trust the WRITER not to be racist. Think about the character Cyborg. Dude is like 90% robot, and just a teeny tiny bit of black man's face. And he keeps being written with the most stereotypical "black man" portrayal stuff! Hell, almost as soon as he appears in Injustice, he gets an "I am a black man" line. It's kind of appalling.
Then again, that raises another point. People would get JUST AS MAD if you turned Cyborg white!
Now, I don't think anyone is willing to argue that Edris Elba didn't do well as heimdall, or even that his costume wasn't as good as comics-version (normal heimdall looks like balls, you guys), and yes, the whole "no black hobbits" thing was pure racism, even if it was racism fueled by unfortunately coherent points. So maybe this guy (i'm not familiar with the actor or his work, except to know that calling him a "star" in Chronicle is reaching) will do awesome as Johnny Storm. But it's not inherently racist to be put off by the actor.
Okay, I've gotta go on another Captcha rant:
I just got a fucking Captcha that QUIZZED me. Not only is it yet another ad on this already-overladen site, but its one that expects me to know when a fucking TV show (which, oddly, i HAVEN'T seen advertised on this site, or anywhere. I've literally never heard of it before) comes on television. Why am I back here in the post box? BECAUSE I GOT THE ANSWER WRONG. And guess what? I have ANOTHER one waiting for me at the end of this post attempt! Expect this rant to continue if the Hampton Inn doesnt offer "better business travel"