Though much of his work is categorized as lyric poetry, Longfellow experimented with many forms, including hexameter and free verse. His published poetry shows great versatility, using anapestic and trochaic forms, blank verse, heroic couplets, ballads and sonnets. Typically, Longfellow would carefully consider the subject of his poetic ideas for a long time before deciding on the right metrical form for it. Much of his work is recognized for its melody-like musicality. As he says, "what a writer asks of his reader is not so much to like as to listen".
As a very private man, Longfellow did not often add autobiographical elements to his poetry. Two notable exceptions are dedicated to the death of members of his family. "Resignation", written as a response to the death of his daughter Fanny in 1848, does not use first-person pronouns and is instead a generalized poem of mourning. The death of his second wife Frances, as biographer Charles Calhoun wrote, deeply affected Longfellow personally but "seemed not to touch his poetry, at least directly". His memorial poem to her, a sonnet called "The Cross of Snow", was not published in his lifetime.
Longfellow often used didacticism in his poetry, though he focused on it less in his later years. Much of his poetry imparts cultural and moral values, particularly focused on promoting life as being more than material pursuits. Longfellow also often used allegory in his work. In "Nature", for example, death is depicted as bedtime for a cranky child. Many of the metaphors he used in his poetry as well as subject matter came from legends, mythology, and literature. He was inspired, for example, by Norse mythology for "The Skeleton in Armor" and by Finnish legends for The Song of Hiawatha. In fact, Longfellow rarely wrote on current subjects and seemed detached from contemporary American concerns. Even so, Longfellow, like many during this period, called for the development of high quality American literature. In Kavanagh, a character says:
We want a national literature commensurate with our mountains and rivers... We want a national epic that shall correspond to the size of the country... We want a national drama in which scope shall be given to our gigantic ideas and to the unparalleled activity of our people... In a word, we want a national literature altogether shaggy and unshorn, that shall shake the earth, like a herd of buffaloes thundering over the prairies.
He was also important as a translator; his translation of Dante became a required possession for those who wanted to be a part of high culture. He also encouraged and supported other translators. In 1845, he published The Poets and Poetry of Europe, an 800-page compilation of translations made by other writers, including many by his friend and colleague Cornelius Conway Felton. Longfellow intended the anthology "to bring together, into a compact and convenient form, as large an amount as possible of those English translations which are scattered through many volumes, and are not accessible to the general reader". In honor of Longfellow's role with translations, Harvard established the Longfellow Institute in 1994, dedicated to literature written in the United States in languages other than English.
In 1874, Longfellow oversaw a 31-volume anthology called Poems of Places, which collected poems representing several geographical locations, including European, Asian, and Arabian countries. Emerson was disappointed and reportedly told Longfellow: "The world is expecting better things of you than this... You are wasting time that should be bestowed upon original production". In preparing the volume, Longfellow hired Katherine Sherwood Bonner as an amanuensis.
Longfellow's early collections, Voices of the Night and Ballads and Other Poems, made him instantly popular. The New-Yorker called him "one of the very few in our time who has successfully aimed in putting poetry to its best and sweetest uses". The Southern Literary Messenger immediately put Longfellow "among the first of our American poets". Poet John Greenleaf Whittier said that Longfellow's poetry illustrated "the careful moulding by which art attains the graceful ease and chaste simplicity of nature". Longfellow's friend Oliver Wendell Holmes, Sr. wrote of him as "our chief singer" and one who "wins and warms... kindles, softens, cheers [and] calms the wildest woe and stays the bitterest tears!"
The rapidity with which American readers embraced Longfellow was unparalleled in publishing history in the United States; by 1874, he was earning $3,000 per poem. His popularity spread throughout Europe as well and his poetry was translated during his lifetime into Italian, French, German, and other languages. As scholar Bliss Perry later wrote, Longfellow was so highly praised that criticizing him was a criminal act like "carrying a rifle into a national park". In the last two decades of his life, he often received requests for autographs from strangers, which he always sent. John Greenleaf Whittier suggested it was this massive correspondence that led to Longfellow's death, writing: "My friend Longfellow was driven to death by these incessant demands".
Contemporary writer Edgar Allan Poe wrote to Longfellow in May 1841 of his "fervent admiration which [your] genius has inspired in me" and later called him "unquestionably the best poet in America". However, after Poe's reputation as a critic increased, he publicly accused Longfellow of plagiarism in what has been since termed by Poe biographers as "The Longfellow War". His assessment was that Longfellow was "a determined imitator and a dextrous adapter of the ideas of other people", specifically Alfred, Lord Tennyson. His accusations may have been a publicity stunt to boost readership of the Broadway Journal, for which he was the editor at the time. Longfellow did not respond publicly, but, after Poe's death, he wrote: "The harshness of his criticisms I have never attributed to anything but the irritation of a sensitive nature chafed by some indefinite sense of wrong".
Margaret Fuller judged him "artificial and imitative" and lacking force. Poet Walt Whitman also considered Longfellow an imitator of European forms, though he praised his ability to reach a popular audience as "the expressor of common themes – of the little songs of the masses". He added, "Longfellow was no revolutionarie: never traveled new paths: of course never broke new paths." Lewis Mumford said that Longfellow could be completely removed from the history of literature without much effect. Towards the end of his life, contemporaries considered him more of a children's poet as many of his readers were children. A contemporary reviewer noted in 1848 that Longfellow was creating a "Goody two-shoes kind of literature... slipshod, sentimental stories told in the style of the nursery, beginning in nothing and ending in nothing". A more modern critic said, "Who, except wretched schoolchildren, now reads Longfellow?" A London critic in the London Quarterly Review, however, condemned all American poetry, saying, "with two or three exceptions, there is not a poet of mark in the whole union" but singled out Longfellow as one of those exceptions. As an editor of the Boston Evening Transcript wrote in 1846, "Whatever the miserable envy of trashy criticism may write against Longfellow, one thing is most certain, no American poet is more read".