Two things that mark Blake as a Romantic poet are his heavy reliance on symbolism and the emblematic representations of nature in his poetry. He is a product of the era dominated by the French Revolution and characterized by a rejection of the preceding literary style.
Romanticism has an emphasis on the unsullied understanding of the world -- like that of a child. His most famous collection of poems, Songs of Innocence and Songs of Experience explore the binary between those two concepts and taken on the whole present a few of the world that emphasizes many of the romantic concepts: importance of the individual, power of imagination, power of nature.
Blake was one of the first poets to challenge society in his poems. He was anti-industrialism when industrialism was booming. He believed in thinking for oneself. "The School Boy," for example, reveals how dangerous it is to accept ideas instead of questioning things. Blake's poem "A Little Boy Lost" reflects his dissatisfaction with organized religion.
In addition to the intense display of emotion in his works, there is an otherworldliness to the writing of William Blake. In addition, he is also typically a Romantic as he is concerned with the plight of people.
One reason was his emphasis on self-expression. After all, he basically invented his own explanation of the universe -- an audacious thing to do. His rejecttion of tradition (at least poetic tradition) would thus be another reason. He turned his back on the "classically" inspired poetry that so common in the eighteenth century. He had a strong sense of the innocence of children. He protested political problems of his day. These are just a few of the reasons he is considered a Romantic.