Whitman mentions in one of his own reviews that his book is the result of a "certain kind of transcendental thinking, which some have styled philosophy." I think that Leaves of Grass got the extensive and often times harsh criticism that it did because it was a revolutionary work in the sense that the literary and poetic society had not previously been exposed to the raw and untamed inner thoughts that Walt brings forth in published form. The piece is often referred to as philosophical because it explores areas of the human condition that previous writers had not addresses, especially on such a personal level. Walt expresses his awareness of the impression that he makes by essentially ignoring traditional poetic politeness and decency, but also points out that it is not arrogance that sparks his desire to write freely but the passion that was dealt by nature and exists in all of us.
Whitman is described as "tenderly affectionate, rowdyish, contemplative, sensual, moral, susceptible, and imperious." This statement brings to light many of the themes that his reviewers addressed, whether positive or a negative. He was criticized at great length for his use of punctuation in the poem, which at the time was viewed as incorrect and undisciplined as oppose to artful or thought-provoking.
"His punctuation is as loose as his morality."
Many readers considered the presentation to be offensive and vulgar, "more like the ravings of a drunkard, or one half crazy." The literary community at the time was not used to works that so blatantly dismissed usual poetic structure and "parlour" appeal. Critics had trouble seeing past Whitman's use of slang and inclusion of subject matter that to the general public was deemed impolite and shameful.
The theme often comes up of rough vs. refined poetry, Whitman of course falling into the category of rough, wild, and rebellious. Even his image shown at the beginning of the book is analyzed by critics as being representative of the sloppy style and low verse of the poem to come.
While Leaves of Grass was widely regarded with negative criticism there were also the readers who recognized the revolutionary tone of the piece and admitted to the desire for sensual and unchecked rawness. Looking at the piece from a modern perspective I think that Whitman took the bold first step to make way for more contemporary poetry that is based on realness and human nature instead of regulation and academia.