The principal object, then, which I proposed to myself in these Poems was to chuse incidents and situations from common life, and to relate or describe them, throughout, as far as was possible, in a selection of language really used by men; and, at the same time, to throw over them a certain colouring of imagination, whereby ordinary things should be presented to the mind in an unusual way; and, further, and above all, to make these incidents and situations interesting by tracing in them, truly though not ostentatiously, the primary laws of our nature: chiefly, as far as regards the manner in which we associate ideas in a state of excitement. Low and rustic life was generally chosen, because in that condition, the essential passions of the heart find a better soil in which they can attain their maturity, are less under restraint, and speak a plainer and more emphatic language; because in that condition of life our elementary feelings co-exist in a state of greater simplicity, and, consequently, may be more accurately contemplated, and more forcibly communicated; because the manners of rural life germinate from those elementary feelings; and, from the necessary character of rural occupations, are more easily comprehended, and are more durable; and lastly, because in that condition the passions of men are incorporated with the beautiful and permanent forms of nature.

2. "the spontaneous overflow of powerful feelings" as a new definition of "good" poetry

For all good poetry is the spontaneous overflow of powerful feelings: but though this be true, Poems to which any value can be attached, were never produced  on any variety of subjects but by a man, who being possessed of more than usual organic sensibility, had also thought long and deeply. For our continued influxes of  feeling are modified and directed by our thoughts, which are indeed the representatives of all our past feelings; and, as by contemplating the relation of these general  representatives to each other we discover what is really important to men, so, by the repetition and continuance of this act, our feelings will be connected with  important subjects, till at length, if we be originally possessed of much sensibility, such habits of mind will be produced, that, by obeying blindly and mechanically the  impulses of those habits, we shall describe objects, and utter sentiments, of such a nature and in such connection with each other, that the understanding of the being  to whom we address ourselves, if he be in a healthful state of association, must necessarily be in some degree enlightened, and his affections ameliorated. 

3. "a Man Speaking to Men" as a new definition of a poet

What is a Poet? To whom does he address himself? And what language is  to be expected from him? He is a man speaking to men: a man, it is true, endued with more lively sensibility, more enthusiasm and tenderness, who has a greater  knowledge of human nature, and a more comprehensive soul, than are supposed to be common among mankind; a man pleased with his own passions and volitions,  and who rejoices more than other men in the spirit of life that is in him; delighting to contemplate similar volitions and passions as manifested in the goings-on of the  Universe, and habitually impelled to create them where he does not find them. To these qualities he has added a disposition to be affected more than other men by  absent things as if they were present; an ability of conjuring up in himself passions, which are indeed far from being the same as those produced by real events, yet  (especially in those parts of the general sympathy which are pleasing and delightful) do more nearly resemble the passions produced by real events, than any thing  which, from the motions of their own minds merely, other men are accustomed to feel in themselves; whence, and from practice, he has acquired a greater readiness  and power in expressing what he thinks and feels, and especially those thoughts and feelings which, by his own choice, or from the structure of his own mind, arise in  him without immediate external excitement.

*Decorum

A term in literary criticism, designates the view that there should be propriety, or fitness, in the way that a literary genre, its subject matter, its characters and actions, and the style of its narration and dialogue are matched to one another. The doctrine had its roots in classical theory, especially in the versified essay Art of Poetry by Horace in the first century B.C. It achieved an elaborate form in the criticism and composition of literature in the Renaissance and the Neoclassic age, when (as John Milton put it in his essay On Education, 1644) decorum became “the grand masterpiece to observe." In the most rigid application of this standard, literary forms, characters, and style were ordered in hierarchies, or “levels," from high through middle to low, and all these elements had to be matched to one another. Thus comedy must not be mixed with tragedy, and the highest and most serious genres (epic and tragedy) must represent characters of the highest social classes (kings and nobility) acting in a way appropriate to their status and speaking in the high style.(M.H. Abrams, A Glossary of Literary Terms, 7th edition, p. 61)