What did Longfellow teach "readers of his day"?
The poem that best answers your question is Longfellow's "The Day is Done." Here Longfellow speaks about the difference between the simplicity he craves from poety. He'd like poetry to be simple enough to read aloud. Indeed, he advises:
Read from some humbler poet,Whose songs gushed from his heart,As showers from the clouds of summer,Or tears from the eyelids start;
...And lend to the rhyme of the poetThe beauty of thy voice.
Such songs have power to quietThe restless pulse of care
Not from the grand old masters,Not from the bards sublime,Whose distant footsteps echoThrough the corridors of Time.For, like strains of martial music,Their mighty thoughts suggestLife's endless toil and endeavor