old enemies coming to the Ponderosa to get revenge
the man falsely accused of a crime about to be lynched
one of the Cartwrights falsely accused of a crime about to be lynched
the drifter getting hired with a mysterious past
an old friend of one the Cartwrights who has a mysterious past and has come to cheat or help the Cartwrights
a minority who has been wronged by the townfolk getting aid and sympathy from the Cartwrights who teach the bigoted townspeople about tolerance
and my favorite plot device: the evil twin.
These and countless other story lines were repeated in practically every long-running western. I still watch them though.