For Wordsworth, the language of poetry should not differ from that of prose except in so far as poetry utilizes metrical forms (and optionally, rhyme). So, poetry is metrical, but should otherwise be indistinguishable from prose. What genuinely makes something a poem is not its language but its "matter," or content. He quotes a famous quatrain by Samuel Johnson that begins, "I put my hat upon my head," and criticizes it as non-poetic not because it uses plain language, like that of prose, but because its content is trivial and insignificant. On the other hand, he quotes Thomas Gray's well-known sonnet on the death of Gray's friend Richard West as an opposite example, a poem which falls below his (Wordsworth's) standard because it uses "poetic" language which is allegedly stilted and inappropriate:
In vain to me the smiling mornings shine,
And reddening Phoebus lifts his golden fire.
Gray's language is not like that of prose, and this is the reason Wordsworth, ironically perhaps, judges it non-poetic. For a poem to move the reader, it must use language that is natural, unaffected, and as close as possible to the way people actually talk—in other words, in prose, except for poetry being "metrical composition." The only distinction, ideally, between prose and poetry is that the latter is metrical. In terms of actual language or diction (the choice of words being used), poetry and prose should be the same, and so-called "poetic diction" as Wordsworth identifies in Gray, is actually unpoetic because it is artificial and unnatural.