The role of imagination
It was his Romantic belief in the supreme value of imagination which made him a romantic poet.
The imagination, of which Keats poems are truly the fruit, takes two main forms.In the first place, the world of his poetry is predominantly artificial, one that he imagines rather than reflecting his direct experience. Furthermore, Keats has all the romantic fondness for the unfamiliar and strange, and for the remote in place and time.
In the second place,Keats' poetry stems from imagination in the sense that a great deal of his work is a vision of what he would like human life to be like, stimulated by his own experience of pain and misery.
Nature and beauty
Nature is present in Keats' poetry as contemplation of beauty; according to the poet the first apprehension of Beauty proceeds from the senses, all of them, not only the nobler ones, sight and hearing, as in Wordsworth's poetry, are involved in the process.
Beauty is caught by the poet in almost tactic images, in the forms Nature takes, in the colours it displays, in the sweatness of its parfumes.
Few poets have had the sense of physical presence of natural beauty and no other romantic could look at nature in its immediatly sensuos aspect, neglecting the ideal behind the real as Keats did.
But beauty is also productive of a much deeper experience than that of an intense sensual pleasure, as Keats adfirms in the opening line of Endymion, "A thing of beauty is a joy forever"; it introduces a sort of "spiritual beauty", that is the one of poetry, love and friendship.
Physical and spiritual beauty are closely interwoven, since the former, linked to life, enjoyment decay and death, is the expression of the latter, related to eternity.