What is the concept of meter and diction according to S.T. Coleridge in his Biographia Literaria?

In chapters 17 and 18 of Biographia Literaria, Coleridge is primarily concerned with either expanding on or, in fact, refuting the principles his friend Wordsworth had laid out in the preface to Lyrical Ballads some years earlier. Wordsworth wished poetry to take a new direction by using the language of ordinary people, especially those who lived in the countryside, for it was this language he regarded as genuinely and more directly expressive of emotion than the diction used by poets of previous ages. He also stated that poetry should be as much like prose as possible, insofar as the poet's word choice should conform essentially to the way people spoke, rejecting affectation and specialized terms which, again, Wordsworth alleged were typically found in poetry.
Coleridge takes issue with the idea that rustic life is especially "real" or that country people speak a more genuine kind of language than others do. He also states that poetry does not, and cannot, be the same as prose, just as written prose is not the same as conversational speech. In Coleridge's view the building-blocks, so to speak, are the same in poetry and prose—he uses an analogy with architecture—but the resulting structure is different.
The most striking passage in Chapter 18 of Biographia Literaria is a critique of Wordsworth's analysis in the Lyrical Ballads preface of Thomas Gray's Sonnet on the Death of Mr. Richard West. Wordsworth had picked apart Gray's sonnet, contending that most of it is worthless as poetry, precisely because it used typically "poetic" diction, rather than the ordinary, prose-like language Wordsworth intended to promote as proper for verse. Coleridge refutes this view. For him, there are combinations of words that are appropriate to poetry and not prose, and vice versa. He points out that most of the lines in Gray's poem that Wordsworth censured are no less poetic than those he accepted. In forging a new path for poetry, Wordsworth seemingly felt it necessary to adopt an extremist view which would have consigned to the rubbish bin not only this particular sonnet of Gray but thousands of other great poems.
Coleridge, in his discussion of meter, gives a more subtle and far-reaching description of the essence of poetry. In some sense, his view is simpler than Wordsworth's, because he regards meter as an element that "tends to increase the vivacity and susceptibility both of the general feelings and of the attention." The metrical construction excites the emotions and causes a more heightened response in the reader than prose would, even if it expressed the same thoughts. But meter, Coleridge makes clear, is by itself just a means to an end, and it is appropriate "only because [the poet uses] a language different from that of prose." This is quite the opposite of Wordsworth's view. Though Coleridge's analysis is lengthy and complex it really boils down to the common-sense view that striking and original diction, combined with meter and rhyme, form the essence of poetry.
It is ironic that although Lyrical Ballads spearheaded the Romantic movement in English verse, not only Coleridge but the poets of the next generation disagreed with Wordsworth's dictum about the ideal poetic language. Wordsworth did succeed in making his own poetry sound like speech, like the way people really talk. Yet there is not necessarily a contradiction between Wordsworth's goals and those of the others. Coleridge's best-known works today, The Rime of the Ancient Mariner and Kublai Khan, are written in language that differs from speech (especially in the former with its deliberate archaisms) but still sounds like natural, unaffected English. The same is true of the younger Romantics: Shelley, Byron and Keats. The corrective to Wordsworth supplied by Coleridge in Biographia Literaria, though it may not have directly influenced these men, conforms to the majority of the poetry written in the Romantic period, and indeed throughout the overall history of English poetry.