I remember Viggo insisting once that his accent alters in LOTR because he's a man throwing off his disguises (the twangs, the moments he sounds Irish or even vaguely Northern English) and accepting his role and place in Middle-earth. Though I rarely think he makes bad decisions...Viggo, man, no one caught that but you. If it was something that helped him with his performance, that's cool, but it does make it seem like he can't stick to an accent.
He has a great voice when he's playing things normal. That whispery thing he favors would, I think, have been an interesting Zod rather than the bombastic thing we'll probably get. Not that a big booming voice is bad.
Oh, and I'll go ahead and throw in one of my favorite stereotyped villainous actors:
No one does disdain like Isaacs. Why aren't you kneeling? He told you to kneel.
