Often, the radicalism of Walt Whitman is emphasized by contrasting his work and biography with those of his contemporaries, the ‘Fireside Poets’. Although there is some validity to the comparison, it should be notes that Longfellow, for example, was engaged in many important prosodic experiments and Whittier was an avid abolitions; thus the Fireside Poets are far more interesting than they appear when simply presented as foils to Whitman.
One major area of contrast was prosodic. Whitman used free verse and the Fireside Poets used regular meters.
In politics, Whitman was generally more radical than the Fireside poets.
In background, the Fireside Poets were well educated members of the upper or upper middle classes whose poetry was published in established journals and by leading presses, whereas Whitman was a self-published poet from a poor background.
Finally, Whitman was gay, and his sexuality and personal life are important parts of his poetry, whereas the Fireside Poets wrote on less intensely personal topics and had less unorthodox personal lives.