Mainly we have two themes in this poem. First one is poetry itself. Through a revolutionary idea Wordsworth changed the entire concept of poetry . Neglecting the lives of kings and queens and lords and ladies as the theme of poetry ,he introduced simple objects as the theme of poetry. This can be called as the Romantic Revival in the history of English Literature. Leaving all the glorious things in the world he took a lonely reaper as the subject of this poem.
Secondly we can say that sorrow and lonliness as the other themes of 'solitary Reaper'. The very oppening of the poem itself justifies this theme: "Behold her, single in the field,". The beauty of loneliness is discribed here. When he says, "And sings a melancholy strain;" we are sure about the theme of sorrow here