The previous post identifies very strong themes of Romantic Thought in Keats' work. I would like to suggest two other elements. One critical element of Romantic thought that can be seen in Keats' work is the idea of nostalgia for a past era. Keats' had a fascination with the Greek culture and the Classical notion of "the good, the true, and the beautiful." We can see this in his analysis of Chapman's translation of Homer and it is an evident idea in "Ode on a Grecian Urn." The idea of using the Greek culture as a mirror for our current setting, reflecting from where to where we need to progress or how far off the mark we truly are is an element that is quite profound in Keats' work. Along these lines, the Romantic hope of being able to expand the moral imagination to embrace a vision of what can be in juxtaposition to what is might be another critical element of Keats' work. Once again, this can be seen in "Ode on a Grecian Urn," where Keats implies that the most profound elements of philosophical thought can be evident in an object that allows us to gaze into what can be from what is. The transformative element of human imagination is another critical element in Romantic philosophy, and is something embodied in Keats' work.
In this response to your question, I would like to comment on two characteristics of Romanticism in Keats's poetry:
1) Preponderance of imagination and feeling over intellect and
2) Sensuousness and pictorial quality.
I have chosen Keats's Ode to a Nightingale as the primary text for this dicussion.
The poem begins with an acute sensation of pain, paradoxically born of an excess of happiness induced by the song of the nightingale:
"My heart aches and a drowsy numbness
Pains my sense........."
As the feeling of pain subsides, the poet senses like gradually dissolving into forgetfulness:' one minute past, and Lethe-wards had sunk '.
What follows from stanza 2 is the poet's desire to migrate to the beautiful world of the nightingale: the typically romantic desire to escape into an ideal world away from the agonies of the lived existence. It's an imaginative excursion for which neither the cool wine long-preserved underground, nor the fresh & hot wine of the South of France would be an appropriate/adequate mode of transportation. Hence the poet at last opts for 'the viewless wings of Poesy', i.e. the visionary flight of imagination to be identified with the bird, and 'fade far' into the dim dark forest.
The nightingale symbolises a remote world of beauty, and only imagination and feeling can lead the poet to that world. However, the word 'forlorn' tolls like a bell in line 70 to make the poet conscious of the limits of imagination and the pendulating return to the lived reality.
Keats is unique for his sensuousness and pictorial quality. His desire for a life full of sensations is wonderfully verbalised in his imagery and phrasing. Consider, for example, the graphically detailed out drinking vessel that he desires to be filled with 'warm South'. Colourful visual details seem to mythologise the 'beaker': ' full of the true, the blushful Hippocrene', 'the beaded bubbles winking at the brim' and 'the purple-stained mouth'. Further on, as the poet enters the deep dark forest with the bird, the fragrant interior full of the odours of the musk-rose, the violets, the white hawthorns, and the eglantines is imagined as an 'embalmed darkness'.
You can also look for similar romantic motif of a journey from the real to the imaginative, as well as romantic sensuous images in such poems as Ode to Autumn, Ode on a Grecian Urn, Bright Star and so on.
John Keats was a Romantic poet par excellence.The two most important Romantic qualities of his poetry,are a love of Nature and a sense of beauty to which a strangeness has been added.Keats' love of nature is best expressed in the sonnet 'To one who has been long in city pent'.The finest example of how when strangeness is added to beauty it becomes Romantic in essence is the lines from 'An Ode to a Nightingale'-" The same that oft-times hath/ Charm'd magic casements, opening on the foam/ Of perilous seas, in faery lands forlorn."It appeals to our imagination and forces us to visualise fairy castles of mermaids on a foamy sea,whose windows are slowly opening to let in the song of a nightingale from the world of men.We miss this quality in other romantic poets.