Essentially, Wordsworth denied that there should be such a thing as a diction specific to poetry. He thought that artifical poetic diction used by many writers obscured the sentiment and feeling that ought to be the focus of poetry. Rather than ornate, basically ornamental language, Wordsworth thought the diction of prose and the diction of poetry should be the same:
It may be safely affirmed, that there neither is, nor can be, any essential difference between the language of prose and metrical composition.
Wordsworth went even further, asserting that poetry ought to be written in the "language really spoken by men," which would accentuate the emotive power of the works by giving them more authenticity. In short, he hoped to strip away what he saw as the pretensions and stuffiness of poetry as it had been written by his predecessors, and his views on diction were central to this project.