In light of the main features of Romanticism, how can I criticize or analyze "The Solitary Reaper" by William Wordsworth?

The power of the imagination was a significant theme in Romanticism. With Wordsworth, this faculty was often tied to having an emotive experience in nature. Finding spiritual and/or profound connection with nature is also a prominent theme in Romanticism. And such experiences for Wordsworth tended to be more profound if he was alone. Granted, he had many powerful experiences in nature which he shared with his sister, Dorothy, Coleridge, and others. But poems like "Lines Written a Few Miles Above Tintern Abbey" and "The Solitary Reaper" illustrate how the individual can use his/her imagination to have an emotional and powerful experience in nature. 

With "The Solitary Reaper," Wordsworth shows how the imagination can transform an ordinary event into something extraordinary. This is also a hallmark of Romanticism. The speaker in the poem is watching a "Highland" (Scottish) girl literally working with and in nature. 

Alone she cuts and binds the grain, 

And sings a melancholy strain...

It's as if she is part of nature's landscape. 

In the second stanza, the speaker starts making this ordinary event into something extraordinary. He lists other powerful songs and sounds in other places and times in history. None compare with the song he hears from the Solitary Reaper. 

In the third stanza, the speaker says he does not know what she is singing about. Therefore, he creates his own meaning. In his imaginings, the song is something probably more profound than what she is actually singing about: 

Perhaps the plaintive numbers flow 

For old, unhappy, far-off things, 

And battles long ago...

In the final stanza, as he leaves, the music stays with him even after it is no longer audible. Wordsworth would often use such an experience and reflect upon it after it occurred. In his "Preface to Lyrical Ballads," Wordsworth describes this idea of composing poetry while reflecting upon such an evocative experience: 

I have said that poetry is the spontaneous overflow of powerful feelings: it takes its origin from emotion recollected in tranquility: the emotion is contemplated till, by a species of reaction, the tranquility gradually disappears, and an emotion, kindred to that which was before the subject of contemplation, is gradually produced, and does itself actually exist in the mind.