What are some common influences and themes seen in his poems, and is there anything uniquely "American" about his poetry?
Some of the particularly American themes seen in Whitman's poetry have to do with the spirit of expansionism that infected the American psyche at the time of his writing. The push west and the great feeling of growth and liberty it engendered were present in much of his poetry.
The form of Whitman's poetry was often in simple plain verse and very plain diction as well, differing greatly from the romantic poets and their sense of rhyme and meter and often relatively complex forms of verse. His distinct plainness was also considered a reference to the American feelings of practicality and simplicity, also common themes in Whitman's work.