Since when does race determine acting ability? People shouldn't be pushing for Donald just because he's black anymore than people should be saying no to him because he's black. This is all so ridiculous.
Walk like you're walking through Hell in gasoline-soaked pajamas.
If Spiderman was an original script, or if it was based off of a book and in that book the character was never given a particularly detailed physical description, then I would be perfectly happy with someone interpreting Peter Parker as a black man. Or Indian, Hispanic, etc. But he wasn't. He was based off of a comic book, with pictures, and the comic shows us more or less what Peter is supposed to look like.
That's why most people would have an issue with it, plain and simple. Because they've read the comic or watched the cartoon, they've seen pictures, and they'd like the actor playing him on the big screen to somewhat fit the role they've grown up seeing. If Harry Potter was played by a blonde guy it would be wrong, because that's not how he was described in the books. If Gandalf's role was given to a 30 year old that wouldn't fit. If If Luke Cage was played by a white man, that would be wrong too. Because that's simply not who the characters are. It's not how they were written. It has nothing to do with racism, ageism... hair-color...ism; it has to do with fans wanting the characters to fit the roles as they were written.
It's a completely separate issue from there not being enough black heroes in film in general, which I agree with. Casting directors should try to look beyond their "handsome white man will make the most cash for our studio" stereotypes while casting original pieces, or should look into making more films about comic or novel heroes of color.
But when there's source material, stay true to the source material. *shrug* That simple.