I'm not sure if you're talking about American literature during the second half of the last century or world lit; so it might be safer to focus on America Lit. since there are several differences between 1900-1950 American Lit (Modernism) and 1950-present day (Contemporary or Postmodernism). One of the major influences of the last half of the 20th century on literature is the widespread nature of diversity during Postmodernism. Whether it be authors from diverse backgrounds (many immigrants found their voice during the second half of the century and many who were once considered minorities published prolifically during the time period) or whether it be diverse genres, the Postmodern era has ushered in a plethora of unique voices and forms.
Another immense influence of the second half of the century upon literature is the emphasis of current events upon writing. Nonfiction has become more popular with the average reader than ever before. Other literary forms that focus on reality and nonfiction such as "faction" and personal essays have either developed or been reborn.
As far as American themes go, in the second half of the century, writers were generally more optimistic than Modern writers. Although postmodernists do not have the rose-colored perspective of early Romantics, they generally tend to focus on more positive themes than Modern authors like Steinbeck and Hemingway did.
There will be many answers to this particular question. I would submit that one of the impacts of the second half of the 20th Century on literature was to increase more voices in the lexicon, within the discourse. In literature, the growth of the narratives of African- Americans, the voices of Latinos, as well as the understanding of gay/ lesbian/ transgendered experiences, and the greater comprehension of women's issues emerged out of the second half of the 20th century. Thinkers like Toni Morrison, August Wilson, Lorraine Hansberry, as well as Cesar Chavez, Richard Rodriguez, Anita Desai, Salman Rushdie, Arundhati Roy, or Joyce Carol Oates, Gloria Steinem, in addition to Tony Kushner, Eric Schlosser, or Rachel Carson all sought to increase more voices and newer experiences into the discussion of what it means to be included. This growth of vocality helps to mark the impact the second half of the 20th century had on literature. Another reality that had a profound effect on literature was the impact of technology on both literature and its production. The emergence of books on taps, electronic mediums, and the online bookstore helped to change how books were written, read, and distributed.