Some of the people crying white washing are justm parroting crap other people have said. There is plenty of racism, sexism, homophobia, ect in this world. There are plenty of bigots in this world (here in the USA claiming that they are "being persecuted for having a different opinion") But Benedict has been someone who has consistently spoken out against (whether he was kidding or not, his suggestion about smoking out all of the bigots by making the next two Presidents be a woman and a gay man to smoke out the rest of the bigots was a great idea). He performed a marriage for a same sex couple. He's worked to help people with various afflictions. He even expressed his less than favorable opinion of a certain name for his fans because he doesn't want to use a certain word to describe women.
With so many actual bigots in the world, people don't need to be slamming someone who is clearly the opposite of bigoted like the Batch. He's a white guy who was cast as a white comic book character. Sure they could have gone with someone not white and it would be fine, but it's certainly not a disaster that (should he take the part) he'll be great. If he was a terrible white actor (like Tom Cruise or the cast of the Expendables) who got the part over someone who was a terrific actor who happened to be Hispanic, Oriental, ect then the calls of white washing and racism would be justified.
As far as those who parrot the demand for more diversity everytime they see another white character on screen, why don't they seek out and support films and TV shows that have characters who are not hetro male WASPs as the lead. They exist, but unfortunately studios don't think there is a demand so the films get art house distribution with little publicity. Why don't people try seeking out and promoting those films? They could tweet and tumble links to the websites for the films and they dates they will come to various cities. That would be showing that there is an actual demand for diversity and not just a bunch of attention seeking parrots on tumblr.
I hope that Benedict will be in parts 3 and 4 of the Avengers. I'm always wanted to see him work with Joss Whedon. I want Loki to be there too, then we can watch Benedict go head to head with Hiddles, RDJ, Chadwick Boseman, Chris Evans. Hemsworth, Anthony Mackie (Falcon needs to be in the Avengers, he is too awesome) ect. That would be epic.