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.
When I saw the movie, I wasn't familiar with the actors, so it was easy for me to imagine that they were Gatsby and Daisy, Nick and Tom. Karen Black, though, as Myrtle was too over the top for me.
There was also an A&E television version. I remember it as Paul Rudd as Gatsby, which would really have been miscasting, but actually he was Nick, and wasn't awful in the role. Gatsby was Toby Stephens, an English actor who maybe did have that "came from nowhere and shot to the top" quality. Daisy was Mira Sorvino -- not quite right.