It's...interesting if unexpected, Then again I don't know what Iron Man is meant to look like without the suit on but if he's going to be IN the suit...hmmm, I'd have thought someone younger and bulkier?
In the comics, and without the suit, Iron Man is a former arms-dealer in his mid 30s called Tony Stark. He is a rich businessman with a moustache. He had something of a drink problem, and is not at all bulky.
I think that this is a great piece of casting. He seems perfect for the role. If you look at his own life story and that of Mr Stark, their are a lot of similarities. You could almost say that he has been method acting the role.