Whitman was a controversial figure because he broke with so many poetic traditions. He wrote in free verse, which was revolutionary at the time and cast him as a prophetic figure for future poets. In many ways, he was about 50 years ahead of his time.
Whitman met with significant criticism during his career because his work was often considered too sexually suggestive; he wrote about "the procreative urge of the universe" as freely as any other subject in his work. Many years after the original publication of Leaves of Grass, the district attorney of Boston deemed it "obscene."
In many ways Whitman is the most "American" of poets because of his trust in the individual and his celebration of democracy. His poetry embodies these ideals because it is completely fresh and unencumbered by "rules."