As the previous educator mentions, the poem is a catalog of all of the cruelties and acts of meanness that human beings inflict upon one another.
Whitman's narrator makes note of the pain of former soldiers ("secret convulsive sobs from young men, at anguish with themselves, remorseful after deeds done"), mothers misused by their children, wives misused by their husbands, the pain of being unloved or of being jealous, and broader suffering—famine, pestilence, and tyranny.
I disagree, however, with the notion that the poem is completely negative. Yes, it contemplates suffering in numerous forms, but it acknowledges the suffering of those who frequently went ignored, such as women and "negroes," who are regarded as victims of the "slights and degradations cast by arrogant persons."
The narrator does not act to alleviate anyone's suffering ("I sit"), but each line begins with an acknowledgement of that suffering—"I hear," "I see," "I mark," and "I observe"—which is the first step toward action.