Maybe. There was a lot of opposition to casting Mia as Daisy.
But for many of us who saw the film, it's hard not to think of the book in terms of the movie. However awful Robert Redford may be in however many ways, he did make the role his own, at least for me.
I'm not sure about Tobey McGuire as Nick in the new version, though. Come to think of it I'm not sure about Tobey McGuire as Spiderman, though at this point it's hard to accept Andrew Garfield in the role.
I read the book (a couple times) before the movie. I think that Redford was already too much of an established "A-lister" and too familiar for a character who was supposed to have suddenly shot to the top from a murky, obscure past; Redford would have made a better Tom Buchanan.
You're right on about Toby McGuire. He's too much a whiney, snively brat. Carraway was something of a stoic, traditionalist with midwestern sensibilities. I'll be very surprised if he pulls it off.