I've heard the argument Bob is making before and feel it is predicated on false pretenses and actually does more harm to the population that it is intended to "help".
A little background on my perspective: I'm a "white-ish" American. I have some Japanese ancestry, but it manifests in a relatively subtle ways like eye shape and skin/hair undertones. I typically check the "other" or similar box when listing heritage, but if its not there, I'll go with Caucasian.
Since Bob made his point rather quickly, I'll do the same and do the exposition later. Injustice or inequality in the past doesn't justify injustice or inequality today, stop blaming slavery and reducing the issue to a "white vs nonwhite issue", institutionalized discrimination has been remedied in the past 50 years, and acting like this will only foster more racism in the future.
Now that the TL;DR set is finished, some exposition.
Lets start with "Thor" itself and examine the issue of the role at hand. If we're going to champion equality, then either ALL mythos should be open to interpretation or none of it should be. "Thor" is based on Norse legend and the people who created these religious figures did so in their own image - exclusively white. Heimdall is not just a race neutral character, "whiteness" is written into it. Diverging from this offers a choice - either we accept that any actor can be chosen for any role and "reinterpretations" are valid, or we don't. If we accept that a black Norse God is viable, we should also be willing to give a white Kunta Kinte a chance - but we don't. A hip-hop reworking of Shakespeare's work is consistently called "daring and fresh", but could you imagine the outrage that would be sparked if a race-reversed Roots was made? The very same people that say "Get with the times, why can't we have a rapping Othello" would rage about "They're stealing our culture! They're polluting a powerful black film!" (Related: Inner city non-whites who lament whites who are "stealing" their culture, yet constantly borrow and express extreme interest in caricatures of Asian culture - Afro Samurai etc...). Bob supposes this double standard is fine, but if we value equality it simply isn't permissible. More on that later.
Before I diverge from film/media to a larger philosophical perspective, I'd like to address what Bob said about "Tyler Perry" actors. The supposition that each one of them is being held back by "The Man" is laughable. This isn't the 20's any longer, non-white actors have a plethora of roles available, both for roles that are written with a certain ethnicity in mind, and those that are not. The uncomfortable truth is that stereotypical roles are MONEY. For the same reason that Nick Cage takes whatever shitfest will pay for the solid gold pyramid he built in his back yard, some non-white actors are willing to play stereotypes because there is a large market for it. Those Tyler Perry movies aren't popular because its the only place black people can go to see those of their own race in cinema, but rather because a certain subset of (predominantly black) audience really, really likes Tyler Perry movies. Also, lets not forget that just like whites, there are a lot of character actors - sometimes a guy plays a gangbanger or a fat computer nerd because that's their appearance and/or range. Every actor in a stereotypical or "light" role isn't a master of the craft who would have a spot in the Royal Shakespeare company if only given a chance. Propagating the whole "The Man is keeping them down" thing is ignorant, especially within the realm of cinema.
Now onto the larger discussion at hand. Bob postulates that having these double standards is viable and fair, largely because of slavery and past injustices. I contend it does more harm than good. This logic simply continues the cycle and fosters racism. Yes, there was a period in history where Western Europe's colonial oppression and racism negatively affected many peoples, but lets not pretend that everyone was living in harmony before evil whitey came and started taking slaves - rarely is it taught that much of the slave trade was sparked by Africans selling other Africans they took as slaves to Europeans, who paid a lot better with exotic goods from the "developed" world of the time.
Over time, we've made a lot of progress in eliminating institutional racism - Jim Crow and the like have been gone for over 50 years. There are generations of people of "minority" races born today that have never suffered from /institutionalized/ discrimination. Note that anyone, regardless of race can be discriminated against by an individual bigot. You'll never be able to quell it completely no more than you can create a society devoid of obnoxious assholes, but you can minimize it through ensuring there is no institution that rewards such bigotry. Part of doing so is eliminating a victim complex where the temporal, yet very real, misfortunes of a group of people is etched into society permanently as a scapegoat and shield of sorts against all future criticism. Since I'm already in my flame-retardant suit, I'll invoke "the big one" and speak of Israeli policy. There are elements in the Israeli government who counter any criticism about their unlawful settlements and extreme use of force with "REMEMBER THE HOLOCAUST", which is echoed here in the USA by AIPAC lobbyists who state that Israel and Jewish people as a whole are generally beyond reproach because of the injustice they suffered during Nazi Germany's reign. This kind of blatant exploitation is paramount to exploiting tragedy and hardship suffered by others tangentially related to oneself, to get one's way through a trump card; changing the argument from whatever its really about to a culturally sensitive issue.
There are many people today who claim they wish "equality" but what they really are looking for is "My turn on top and to punish the people who were previously on top". This is the crux of the issue. These double standards only harm in the long run. Nobody wants to hear that some of the only racial/gender discrimination that is still institutionalized as well as implicitly allowed, is against white males. In the past 20 years, those of us growing up in a world that for all intents and purposes has equality before the law in terms of race, but due to a misguided attempt to inform about past injustice, a present one is created. I can remember throughout secondary school, besides the typical classes present, there was a huge push to provide "diverse" literature. Normally this would be a good thing, but the implementation was nigh exclusively stories of non-whites struggling against brutal oppression, often caused by whites. This is a common theme in media today and especially compounded by tales handed down from older generations of non-whites, who may have experienced institutionalized discrimination, sets up prejudice in a younger generation against their peers.
A good place to see this is in collegiate quotas and scholarships - "generic American" White males have to compete for a relatively small amount of merit-only and need-based scholarships, while other backgrounds are not only often given preference in financial need-based programs, but have special programs and scholarships exclusively for them. Compound this with college quotas and desires to be "diverse" and you have a considerable hardship on the vast majority of white males, because of the baseless belief that all white people are wealthy enough to pay for college on their own. I've personally been affected by this in a major way. I attended a Top 5 college for my program of study and was awarded the highest merit scholarship available. It came with full tuition, room/board etc...a true full ride plus a stipend and special exclusive seminars and leadership/networking conferences. When I showed up to the orientation, I noticed something wasn't quite right - Myself and an Asian fellow were standing around looking a little perplexed. We were soon taken aside and told that a mistake had been made and that this scholarship was only available to Black, Hispanic, and American Indian students, or other "disadvantaged" minorities In deference to the mistake, we were both given the highest academic scholarship, but that only included full tuition leaving us to pay for everything else out of pocket. Is this really fair and egalitarian?
While I'm well aware that there is a limited range of necessity for certain measures to compensate for shifts in policy accompanied with rectifying injustice, when it goes on forever it simply institutionalizes injustice against another party in a tacitly punitive way, often affecting those who weren't even alive to perpetrate nor reap the benefits of the original discrimination. Take for instance Affirmative Action - right after the Civil Rights movement, it was necessary to ensure Black Americans got past the bigotry in the workforce. However, 30+ years later, it serves only to give the unqualified jobs, which spawns enmity in others and creates the perception that "minorities" need an unequal playing field, which is a disservice to hardworking, competent non-whites. There is also the issue of using a policy sledgehammer to solve problems better adapted to a scalpel - we have laws in place to address discrimination, so we need to use them instead of simply painting with broad strokes. Some may say that the legacy of injustice in the past affects things today, but one can only go so far before it becomes a spurious claim that serves to profit by simplifying complex multifaceted issues into polarizing "Its all their fault, they need to pay" rhetoric.
In the long run, such attitudes actually lead to bigotry. How quickly those in the past felt to see others having preferential treatment while their own demographic was told they were unworthy. Doing so to the descendents of the original group only spurs hostility and prompts the cycle to renew. Today we're teaching our young white males that they're the cause of all the wrongs done to non-whites and thus they need to apologize for their ancestors, go out of their way to kowtown to others as reparation, and accept a lower standard of services; to do otherwise or seek true equality is "racist". Subjugating any group of people breeds resentment, nomatter the racial or gender paradigm, but there aren't enough level heads in society to realize this, over those that want "their turn on top". This attitude is illustrated in how many reply to "But now we're being discriminated against, how is that fair?" good "GOOD, now you know how it feels!" which is as despicable as it is erroneous.
With respect to the discussion just about media, I think there is enough room for both new interpretations and traditionalist approaches, however people should have the right to comment that adding a token black actor playing a Norse god isn't quite the same as a retelling of The Brothers Karamov with a Latino cast. That said, this discussion is more than just about movie casting, but if double standards benefiting certain demographics belong in a society that values equality. My opinion is a hearty "No", and that instead of swinging the pendulum to extremes of oppression from one side to the other, it should be stopped squarely at the midway point - equality - lest there be an eternal cycle of discrimination.