While I'm not a fan of PC-ifying stuff, I find it laughable that some people are saying "well this is based on Avengers original comics"...except for Black Widow and Hawkeye being founding members, and Hulk not immiedetly leaving the team after the first movie. And even of those two characters, Hawkeye's characterization is much more similar to Ultimate Hawkeye than the traditional 616 characterization or the more recent Fraction/Aja characterization. Also the heavy presence of SHIELD is certainly not in early Avengers. And that's not even going into the team's individual origins, which takes at least some liberties with the origin in the comics.
I think some of the Marvel Movie folks are afraid of pushing the envelope too hard. It has been noted by comic and cinematic sites that Marvel certainly has a "formula" for its films, and I think a part of that formula is leaving female or minority focused movies for the late phase 3. Avengers 2 certainly made some headway into diversity, with eastern European and African Americans being on the team (I don't know if Scarlet Witch's actress actually is eastern European but she certainly does a better accent than Black Widow).
In contrast, we're getting a lot of diversity out of the live action TV division. Agent Carter is back for a second season. Agents of Shield has Wen and Bennett as Chinese Americans and playing Chinese American characters, and the roster has had African Americans too. The show still has trouble with the women getting much more brutal, torture like treatment more ofthen than the men but I'm hoping they correct that. Luke Cage and Jessica Jones will be on Netflix next month, and I'm hoping that Iron Fist will be a bit more...diverse than the whitewashing that is likely to happen with Dr. Strange.
I think now that we're out of the way of Iron Man 1 being a fluke, Marvel will take more chances on diversity in terms of film. Unfortunately Hollywood can still be a tough place to break that barrier, even in the modern age.
You don't have to worry about Dr. Strange. They've hired a black actor to play Baron Mordo. So, you know, there'll be some blackwashing too.