Personally, I don't find "race bending" characters a huge issue, save when a certain character is definitively of a given race, for whatever (i.e. their race is part of their identity, etc.). For me, gender bending is almost always a no-go--I have no problem with a cast that has leads with both genders (who would?), but changing an established character's gender to suit your film's tone shows a lack of creativity, IMO, as you're choosing a tone or atmosphere over your characters, who should drive your film.
TBH, what irritates me more than gender or race changes is when a movie based on pre-established characters ignores the fundamental traits of those characters. Take, for example, Homecoming's Flash Thompson. I could care less that he's played by an Indian actor, but my goodness is he scrawny! Flash is supposed to be the stereotypical jock--big, bulky, and brainless. The casting choice they made here doesn't seem to fit that, IMO. Sure, there are both intelligent and thin jocks, but Flash is not one of them.
Nevertheless, I still think this Spidey film is going to be the best yet, because I'm confident in Marvel's skill. They've surprised me pleasantly with their character interpretations before, so while I might be circumspect about a few of their choices here, I'm reasonably certain that they'll bring something impressive and new to the table.