I'm sure it would have been much grittier. And by that, I mean a lot more urban; more shots of the city, etc... One of Scorsese's great strengths has always been his love affair with the City of New York. To be more precise, the dark side of the City of New York.

Plus, I think we would have seen more of the Mafia at its street level, as in "Goodfellas" and "Mean Streets." Two of my favorite films of all-time, but they had a much different look than Coppola's films. I don't think it would have worked.