The traditional ballad is a songlike poem that tells a story dealing with romance or adventure. The narrative is usually told in simple language with dramatic action, dialog, and repetition; often there is a refrain.
Literary ballads, such as "La Belle Dame sans Merci," grew out of an increasing interest by the Romantic poets. They follow the form of the quatrain, with the second and fourth lines rhyming. (a/b/c/b pattern). It has alternating lines that are in iambic tetrameter, followed by iambic trimeter. There is, however, a line that are pentameter, breaking form such as in the last stanza, "Though the sedge has withered from the lake." Also, there is no refrain as in a traditional ballad as the only repeated lines are the last lines of the first and last stanzas: "And no birds sing."
Thematically, Keats's' poem succeeds as a ballad because it is a melancholy lament over the impermanence of a life experience; in this case, beauty. And, it deals with the supernatural and death. In fact, the knight does not fall in love with a woman; it is a fairy that he loves,
She took me to her elfin grot....
And there she lulled me asleep,
The latest dream I ever dreamed
On the cold hill's side.
I saw pale kings and princes too,
Pale warriors, death-pale were they all;
They cried--'La Belle Dame sans Merci
Hath thee in thrall!
Keats' ballad is similar to an ancient myth in which the fertility of the land is tied to the health of a heroic figure such as a king or a knight.