Blake was different from his “fellow writers” in many ways. For one thing, he wasn’t just a writer, or even (arguably) primarily a writer. Blake was an artist. His illustrations were not only outstanding artworks in their own right, they also “explained” or provided context for his poetry. So I think this visual aspect to his work is one “mental power” that sets him apart.
Blake also was his own publisher and printer. As a master engraver, Blake invented an entirely new process of making plates that could accommodate his artistic vision. He created and printed his books himself, so they were truly handmade objects. So, I would argue, Blake’s craftsmanship also sets him apart.
Above all else, however, Blake’s unique poetic vision, which—was driven by his unorthodox religious views and his interest in prophecy and visions—set him apart from his contemporaries. Blake anticipated Wordsworth in extolling the spirituality of nature, but his use of symbolism and allegory to express complex interpretations of spiritual history and to prophesy the future (see, for example, The Marriage of Heaven and Hell) suggests a kind of religious intensity and nonconformism that has few equals in English literature.