I gave a brief description of some of the key points of modern poetry in your "Love Song for J. Alfred Prufrock" question, but just to recap on the most significant highlights that would correspond with T.S. Eliot's The Waste Land:
Breaking the rules or conventions of more traditional, accepted forms of poetry:
The Waste Land veers in and out of different languages (German, French, even Hindi, to name a few) and obscure mythological references without clear focus or easily defined theme, without defined meter or form. Eliot's enormously long poem does not fit any traditional structure, and in doing so, Eliot set the standard for modern poetry.
Themes of Fear, Alienation, City Life, War, Ennui, Death
Alienation and the desolation of war are predominant themes in The Waste Land; Eliot originally writes this poem after he suffers a mental breakdown at his bank manager job in London; he felt keenly the destruction and decay of World War I and mourned the loss of innocence and spirituality. He pours those feelings into The Waste Land, and the resulting poem is a dark journey through a broken, devastated kingdom, the perfect metaphor for post-WWI Europe.
The whole first section of the poem, "Burial of the Dead," is thick with references to death and a feeling of imprisonment or being trapped. Multiple allusions to WWI and Germany lurk throughout, from references to Starnbergersee (in Germany) to including actual phrases written in German; Eliot's poem reminds readers of WWI and the bitter price of war:
"Under the brown fog of a winter dawn,
A crowd flowed over London Bridge, so many,
I had not thought death had undone so many" (61-63).