Don Glover (community) made a good point: Seeing Micheal Ceral play 'Shaft" would be awesome to see, but it wouldn't fit the character. He made this in reference to a rumor about himself playing Peter Parker in a role that went to Andrew Garfield.
(I still think Glover should have played Spidey!)
Sometimes it's about the character, and sometimes it's about the continuity. Morgan Freeman played 'Red' in Shawshank Redemption, when the character in the book was an Irish redhead. I couldn't imagine that character being played by anyone else, even someone who might physically fit the role.
When a movie has racial tones (Like the Color Purple, or American History X) you need the character to be a certain ethnicity. If they make the Marvel movie Black Panther I don't think Kevin Spacey is a good fit, even though he's an amazing actor, because that superhero has also had to deal with racism, something that wouldn't translate if everyone is reacting to Panthers skin color and not trusting him based on that (Government officials and the like).
(Peter Parker was picked on for being a nerd, so his character could really be any ethnicity).
Johnny Storm did meet Black Panther when he was in college, so that's really the only story issue that might ever develop ie why are they treating Panther with racial overtones, but not Johnny? Also they may make him adopted or make Sue Storm, his sister, black as well).
“But you would never let Gweneth Paltrow play Rosa Parks in a movie!”
Well, Rosa was a real person. So no, we wouldn't do that.
Fictional or not, some characters face ordeals based on culture. Gwen playing Rosa couldn't work because of, yes, Rosa's story is an ethnic and cultural story. Black Lightning is a fictional character, Steel, Black Panther, too. We wouldn't change those characters race because it is part of their origin. And that's where race comes into a character, fictional or not. Would Rosa's life be different if she had been born white? Yes. Would Peter Parkers life have been different if he was black? No, because of the environment he was raised in; he was bullied, smart, had a hard working family that loved him. Is that something only exclusive to white people?
I for one welcome African-American Johnny Storm. Sometimes the best books don't make the best movies, and certainly some of the best movies wouldn't make very good books. I can think of movies I have seen that were amazing and read the book and said "What the hell is this sh*t!" (My Life Without Me was adapted from the novel Pretending the Bed is a Raft by Nanci Kinkaid, and in my opinion is about "idiot people doing idiot things because they are idiots". Whereas the film is a poignant lesson in life after our own death, our selfish wants against our altruistic motivations, and really heart wrenching).
There have been many changes to superheros when they cross to the bigscreen, and sometimes I am very grateful. I could tolerate the giant blue penis in the Watchmen because of Dr. Manhattan's disassociative behavior. I could not have watched X-Men with Wolverine in tight yellow spandex: those are two very differently motivated characters. And c'mon, in the real world would a cigar chomping Harley riding guy with a metal skeleton and giant claws really wear tight yellow spandex? Not unless he was REALLY comfortable about his sexuality.