Keats's "La Belle Dame Sans Merci" has traits both of the romantic, in the modern sense, and of the Romantic in the sense of the Romantic movement. It is romantic in that it paints a picture of the encounter between a young knight and a "full beautiful" lady who "looked at me as she did love / And made sweet moan." I assume, however, your question is really more concerned with the traits of Romanticism which pervade this poem, which served as inspiration for later literary movements, including the pre-Raphaelites, who admired its focus upon a psuedo-Arthurian past.
References to the past, often without specifics, often occur in Romantic poetry. The Romantics frequently alluded to the classical, but an aspect of dwelling in a nebulous past served to further the Romantic preoccupation with a more innocent time, untouched by industry and modern drudgery. The knight in this poem, certainly, exhibits this sort of naivety, allowing the beautiful lady to lure him with her "faery's song" into her "Elfin grot." Lulled to sleep by the lady, the knight sees a vision of "pale warriors," presumably the former victims of the fairy lady, who tell him he too is now "in thrall."
The Romantic preoccupation with nature, too, and the inspirational beauty of nature, permeates this poem. The knight is drawn into his "thrall" by the "wild" eyes of the lady in the "meads," a creature of nature. Accordingly, he decks her in a "garland" of flowers, and she finds him "honey wild, and manna dew." When the knight has been left alone on the "cold hillside" by the lady, by contrast, the natural world around him is withering: the "rose" and "lily" of the knight's own beauty are in decline, and "the sedge is withered from the lake / And no birds sing." Nature seems to reflect the knight's emotional state, an example of pathetic fallacy.
The emotional response of the knight to the faery has left him "cold" and "palely loitering." However, like the "pale kings and princes" who had gone before him, he is now seemingly left with his new wisdom to act as a new "warning" to others, an example of the poet-as-visionary trope also common to Romantic philosophy.