Well, besides Sophia's obvious bad acting, the rest of the movie was lacking good actors as well, suffering partly because so many of Puzo's already-developed characters were absent.

No Hagen, Fredo, Sonny, Clemenza, Tessio... Yeah, I know they're "dead", but some flashback scenes from the 20s would have been awesome, even with new actors.
Instead we get Michael, Connie, Kay... and nobody else. You can't make a movie out of that.
We also get Vincent Mancini played by Andy whatever; a horrible actor, imo.

Those original CHARACTERS are what made the Godfather... the Godfather.

The vatican storyline was pretty boring too, but still Sophia is my main gripe with the movie, nonetheless.