Lets start of with Colossus.Our first taste of Colossus was in X2:X-Men United when he walked in, like a gangsta and turned metal and laid down some ass whuppin on the government, then he talked and we noticed Colossus, doesn't have the Russian accent, he had in the cartoons and comics.
He wasn't poorly casted it was just where is the accent, we didn't even hear it in X-Men 3 the Last Stand.
Next my personal favorite, Gambit.Taylor Kitsch had Gambits cool, he had his charm(No Homo) but he was lacking one thing, that French accent
I know Gambit as the French card dealing, street hustler and I expected a French accent, I was utterly disapointed.
Now Storm, played by sexy, beautiful, talented Halle.......Berry, but she had that, slight African accent moment in X-Men but, she didn't have the full on African accent, thats why she is considered a bad choice for Storm, she didn't deliver the accent
Black Widow, played by our generations Marilyn Monroe(sorry Mrylin Manson's that goth rocker chick, I meant Marilyn Monroe), Scarlett Johanson.She is sexy and a waaaaaaaaaaaaaaaaaaaaaaaaaaaay better choice for Black Widow than Emily Blunt(Blunt cant act and she isn't and never will be as sexy as Scarlet), but Scarlet, husband of Deadpool and Green Lantern keep up with times give us a little hit at the Russian accent, Rourke did it perfect, but at least give us an accent in the Avengers, please.
Now I'm not putting all the blame on the actors for not providing an accent, I'm blaming the studios, the writer, the director.The biggest issue is Fox Studios and Bryan Singer, Jon Favreau and Paramount are not an issue.