Right. I've been thinking about this (I know I shouldn't, but that's me) and I think I've come up with a better example. It took a while, and that alone tells a story all by itself (and not a happy/good one, either), but...
I know you're a fan of TWD, series and graphic novel, so...how would you have felt if, after reading the comics and having a fixed idea of who certain characters were, what they generally looked like, seeing what the author's (authors'?) original intentions had been, etc, the series producers had then cast Morgan as a white bloke?
There, as with Roland, colour makes no serious difference, but it does go against what is 'known' about the character and therefore expected by fans as and when it jumps media.
Now, I'll say that I expect Idris will knock it out of the park in terms of performance. SK was, IMHO, dead right about his acting ability, and I hope that was actually the only criterion used when it came to making the decision (I doubt SK himself was involved, but if he had been, it's 100% that it would be down to ability to do the job and nothing else).
What concerns me is if something else crept into it, consciously or subconsciously - a reaction to certain recent controversies, for example (yeah, the Oscars, pretty much) - because that's not the path to the real, true equality that all people deserve. IMO.
The bottom line for me is this: excellent actor, decent bloke (by all accounts), wrong role.