It's actually a relatively decent cast across the board, though casting Milla Jovovich as Milady strikes me as an error of understanding of the character: she's supposed to look angelic and sweet at first, only revealing her darker side later in the story. But Mikkelsen is really a standout choice for Rochefort, a hard character to nail since he begins the series as D'Artagnan's enemy, and ends as his friend, and victim. 

I've been slightly obsessed with Mikkelsen since he redefined Bond villains for the modern age as Le Chiffre in Casino Royale. He's capable of simultaneous lethality and intense emotional control, but unlike other action heroes, he doesn't come across as a blank. There are always a lot of things going on behind the facade his characters maintain. I think Rochefort will benefit from his experience in that kind of balance, and it'll be good for Mikkelsen's American career for audiences to see him as someone with style, and a bit of humor.

Of course, I can't divine what the script will be like from casting decisions. It might be an awful adaptation. But I think it shows a certain intelligent sense of the material to cast Mikkelsen, and I also like the decision to cast Orlando Bloom as the well-intentioned, but ultimately weak, Duke of Buckingham. Bloom pulled off his performance as Legolas in the Lord of the Rings movies because the character is intentionally delicate-but-badass, but in Pirates of the Caribbean, it always made sense to me that Elizabeth became the Pirate Queen rather than Will, and that Will made a sacrifice for love. Bloom's more of a lover than a fighter. And Mikkelsen will give the quite young Logan Lerman, who will be outgrowing his Justin Bieberish hair as D'Artagnan, more than a match for his steel.