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
spring when the world is mud-
luscious the little
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.