In the early days, the MPAA almost automatically gave a G to any reissue of a vintage film, even GONE WITH THE WIND and BEN-HUR. Those films are now being rated more "correctly." And yes, MAD would certainly get a PG today. As for OZ, I don't believe the 3-D had anything to do with the new rating; it was because it's a pretty scary film for young'uns. Ratings apply to content, not format.
I still think the parameters of the G rating have shrunk to the point where it's now only the most impossibly innocuous for the most hypersensitive. This December we'll see a movie about Paddington Bear. I can see it right now: PG for "some peril." Geez.