Most of our ratings problems come from the PG-13 rating; that is, nobody seems to agree how far is too far for PG-13 and where R begins. Basically any substantial amount of spurting red blood will about get and R rating, and things like decapitations of non-humans is an instant PG-13. I think decapiting humans get instant R. (PG-13 movies can show removed heads, just not the removal.)
The biggest issue of PG-13 vs R is sexuality, though. For some reason, it seems any amount of frontal nudity will get an R rating, but PG-13 films can have all kinds of raunchy dialogue and show sex scenes if they're "Covered up" ... It makes little sense to me.
As for G vs PG, the ratings have changed a lot since 1977. A movie that gets PG today, say National Treasure, probably would have gotten a G rating in the 70's: There's no significant swearing, nobody is murdered, ect. As for Starwars, I think even without the burnng owen and beru corpses, it would have gotten PG because of all the Rebel pilots killed at Yavin, the force choking, and the destruction of Alderaan. Lucas, I think, changed the Greedo scene because of our societies changing attitudes towards violence--that is to say that good guys should never be the agressors--which of course completely ignores the fact that Han wasn't exactly a good guy, and that even if Han shot first Greedo was still the agressor. Basically it was changed to make it clear it was self-defense and not murder.