Though Wordsworth and Coleridge were co-poets in writing and publishing the 'Lyrical Ballads', Coleridge did not agree with Wordsworth in his theory of Poetic Language enunciated by him in his preface to the Lyrical Ballads. There were two cardinal points in Wordsworth's theory. First, that poetry should be written in the language of the common men as really spoken by them. Second, that there is no essential difference between the language of prose and that of metrical composition. Coleridge did not agree with either of these points.
Regarding the first point he says that there is an obvious contradiction in Wordsworth's own statement. Wordsworth was conscious of the fact that a charge of 'triviality and meanness' may be brought against poetry written in the language of the common men as really spoken by them. Therefore he adds that the language of the common men should be "purified indeed from what appear to be its real defects, from all lasting and rational causes of dislike or disgust." Now Coleridge's contention is that when the language of the common men has been so purified and corrected and improved upon, it no longer remains the language of the common men as really used by them. Therefore there is no rational point in this part of his theory of language. Moreover, all the major poems of Wordsworth himself are written in a language which common men would even hardly understand.
Coming to the second point, Coleridge says that there will always remain an essential difference between the language of prose and that of poetry. A poet has to write in rhyme or metre. This very fact would change his choice of words and structure of sentences. Poetry cannot be written as prose. So long as rhyme, rhythm, and cadence remain essential requisites of poetry, its language will remain different from prose. In conclusion Coleridge says, "there is and will always remain an essential difference between the language of prose and that of metrical composition." 
