I never realized it as a kid, but Johnny Weismuller was a very handsome man. And in top physical shape. As an Olympic swimmer it's no wonder, but still as an actor he brought a charisma and solidity to the character of Tarzan that made him quite likable.
And Maureen O'Sullivan was exactly a slouch either. Her warmth and friendliness shone through the screen and into the hearts of the audience.
Hard to believe that such films could be made then, and it'd be a big surprise if they could be now, since the tendency in films now is to be over the top in realism, which makes them cringefests more often than not. Which doesn't make them very settling for youngsters.