La Belle Dame Sans Merci is a ballad written by Keats on the temptation and tragic disillusionment of love. Composed in the mode of a dialogue, the poem reveals a tale recounted by a knight-at-arms, seduced by a fairy and then left forlorn in a cold hill-side with all the charms of love gone and the nature withering away.
Ode to Autumn is, on the other hand, an ode which emphasises the theme of fruition and fulfilment inspite of the inescapable winter. The poem doesn't betray any despair or melancholy for the long-sustained growth-process of Autumn delays death, and even when death is just about to come, the sights and sounds of Autumn are still no inferior to the vernal audio-visuals symbolising new life.
La Belle tells us a story of deception and despair, failed and aborted love ending in a condition of irrevocable fall. To Autumn goes on to celebrate the value of life, its continual ripening to the point of death, and the renewal after death, the spring poised to come.