Pay close attention to the 1967 Fantastic Four episode, "The Way It All Began", which explains the team's origin for the first time on television. Considering the casting for a reboot of the feature film version of the series, comic book purists will have reason to be up in arms again.
In another example of casting to the attention of casual, non-comics-reading fans, Michael B. Jordan (no relation to you know who) was cast as Johnny Storm in the pending reboot, with Kate Mara as Johnny's sister, Sue. Jordan is African-American. While Hollywood has been able to succeed by recasting white supporting characters as African-American in the transition from comics to screen (Samuel L. Jackson was, in fact, the model for Brian Bendis' reboot of Nick Fury, and Marvel neatly explained that away a few months back in storyline), doing the same for a central player in this film may be a mistake. Ben Grimm aside, there is a reason why the FF is known as the "First Family" of superheroes. By the time the above episode aired, Marvel had already had Reed Richards marry Sue Storm, which makes Johnny Reed's brother-in-law. It's going to be a little difficult to explain that away in this new movie.
I get the rationale behind the selection of Jordan, having previously worked with either a castmate or the director on another project. I get that they are targeting, as noted earlier, the non-comics audience who may have some periphery knowledge of the long running FF franchise. However, it wouldn't surprise me all that much if this ends up backfiring.
"The Way It All Began" merits an A, by the way.