What Keats does in his poems helped to develop the idea of Romanticism. He defined it, in effect, along with several other poets of his period. In other words, your question could certainly be turned around to ask something like: what are features in Keats that demonstrate what eventually became called, Romanticism.
The point is that everything in Keats is romantic poetry. He is a romantic, and everything he does is romantic.
Several important elements of Keats's poetry came to define Romanticism.
- Interest in the lyric, the personal.
- Interest in the transcendent, that which is beyond human understanding.
- Interest in the transcendent as it relates to nature.
- Interest in a glorified or romanticized past, both the medieval and the classical.
- Interest in the role of the poet, as well as in the creation of his art.
- Interest in the imagination, which intuitively connects with nature and the transcendent and develops over time.
The above are what Keats is interested in writing about.