What are some of the major features of the poetry of the twentieth century?

Poetry in English underwent radical changes in form and language, particularly owing to the work of a number of American poets. Where poetry was once written according to agreed upon standards of form and diction, poets in the twentieth century began to experiment with these traditions, and opened up the definition and scope of what poetry was and could be.

Some poets who were writing before the turn of the century did in fact influence...

Poetry in English underwent radical changes in form and language, particularly owing to the work of a number of American poets. Where poetry was once written according to agreed upon standards of form and diction, poets in the twentieth century began to experiment with these traditions, and opened up the definition and scope of what poetry was and could be.

Some poets who were writing before the turn of the century did in fact influence those who came a bit later; these include Walt Whitman, whose writing in major works like "Song of Myself" held usher in a new approach to language that was less formal and more personal and idiosyncratic. Whitman hardly thought of himself as "experimental"--he simply had things to say and very particular ways of saying them, and his passionate, ecstatic mode of poetic voice was controversial and ultimately very instrumental in changing the way poetry was written and regarded.

Some of the most important poets of the last 100 years used an experimental approach to forms and language; these include e. e. cummings, whose work did not use standard capitalization or punctuation. Wallace Stevens was an important and prolific poet who worked his entire adult life as an insurance salesman; his complex, unusual and intricate use of language allowed him to craft very memorable images in his poems, like "The Snow Man" or "Thirteen Ways of Looking at a Blackbird." Stevens often used combined phrases and images in ways that seemed quite illogical and surreal, and yet still made sense grammatically. 

T. S. Eliot was another influential poet; he wrote often using the first person perspective, which was an unusual technique at the time. He would also shift narrative modes within one poem, switching between points of view and settings rather abruptly. He wrote of social and political issues, but usually in veiled language so that his exact perspective was not overt or obvious. William Butler Yeats was an Irish poet who also wrote of social and political issues; he included references to folklore and ancient Irish mythology in his work. 

The twentieth century also saw the rise of poetry written by women; some of it was considered feminist poetry (such as the work of Adrienne Rich or Gertrude Stein) but much of it illuminated the female experience in ways that were disturbing and haunting (as with Sylvia Path's work, that hinted at her struggle with depression and domestic strife), and obviously spoke of deep inequality and sexual double standards. As with other poetic works of this period, the writings of female poets were seen as part of an increasing trend in consciousness raising within education and cultural literacy, and poetry became a way for people to express and share thoughts about social issues undergoing volatile change in the twentieth century.