What are the major characteristics of E. E. Cummings's poems?

There are two aspects of the poetry of E.E. Cummings that make him significant:

1. He was on the "cutting edge" of the Modernist, experimental movement.  His poetry is new in its typography, its syntax (he uses nouns for verbs and vice versa), grammar, and punctuation, introducing audiences to the innovations of verse and prose in the early parat of the twentieth century.
2. Cummings was an effective satirist and intense lyric poet.  His targets for satire are often hypocrisy and the submergence of the individual in the military and in society  because of the "mass mind of the mass market."  His lyric poems celebrate love and the truth of the moment, herald individuality, exalt a child-like love for nature.  In an age that was reserved in its feeling, Cummings was personal and unapologetic about his lustful feelings and individual desires.

Possessing a highly personal and idiosyncratic style, Cummings appeals much to youth in his giving of new life to the ideas that have always been.  His poetry exists in the present,  in the aliveness and pure essence of the state of being without regard to the flow of time.  His poetry is existential and romantic both:

Spring is like a perhaps hand
(which comes carefully
out of Nowhere) arranging
a window, into which people look (while
people stare
arranging and changing placing
carefully there a strange
thing an a known thing here) and
changing everything carefully....

In these lines from the poem "Spring is like a perhaps hand," Cummings demonstrates the existentialism--coming out of Nowhere--and romanticism--nature/Spring that is "like a perhaps hand" that people watch appreciatively through a windo. The odd use of syntax in "perhaps hand" is apparent as is "arranging and changing and placing carefully." 

What is E. E. Cummings' writing style?

First, while it is true that Cummings wrote his own name without punctuation or capitalization (e e cummings), it has become common practice for scholars to normalize the form when citing or referring to him. The oddness of this practice is emblematic of his style; he simplified the verbal expression of his poems as one way to signify the essence, rather than the appearance, of his ideas. Further, his grammar is intentionally contra-normal, again to express something more than simple meaning. For example, when he says

“in Just-

spring          when the world is mud-

luscious the little

lame balloonman”

he is making a subtle point: Spring has a brief but notable beginning, a texture, a “personality.” Likewise, the arrangement of his lines (entirely unorthodox) allows the reader to visualize the delicate moment of Spring (and in the complete poem’s contour, the shape of Bacchus’ hoof). Cummings' style, then, is to alter the conventions of the written language to expose its hidden possibilities. It's important to note that these variations are not arbitrary but carefully chosen.

His larger canon emphasizes as well his attention to the details of individual personality, his personal optimism, and his love of nature (especially the seasons, the seaside, and the trees).

Perhaps his best "style" is his self-effacing personal viewpoint. He once remarked that English is the only language that capitalizes the first person singular nominative pronoun.