This is not an easy question. Kushner's drama has so many levels and so many intersecting ideas with the different characters. Harper might have one of the better lines in the drama that talks about one of its main ideas: "But everywhere," she says, "things are collapsing, lies surfacing, systems of defense giving way." Indeed, Kushner's question that plagues at all of the characters is how does not live their life in a condition where so much is "giving way." This is a sword that cuts in both directions and helps to comprise a significant idea in the drama. For many of the characters who are gay in the drama, this issue is present with the condition of AIDS that is taking such a toll in the gay community. Prior Walter struggles with this, as does Belize, and even Roy Cohn. Each character must deal with the concept of collapse on physical, emotional, and professional levels. The reality of AIDS, its death and sickness, as well as the transformation it prompted compelled many to have to engage in difficult reflection about one's identity in the world and how they have to play a role in constructing it amidst so much that is "giving way."
For other characters, the defenses of the past are "giving way." Louis has to endure this crucible, as his tendency to politicize everything and create barriers between himself and the awful truths that exist at the core of his being have been his dominant truth. Through his relationship with Joe and having to come to terms with what he did to Prior, Louis must learn to relinquish his desire to politicize and "intellectualize" reality in order to live in the present and the future. For Joe, his past in believing that narrow emphases of social, political, and spiritual focus can eliminate uncomfortable questions end up "giving way." Harper must learn to embrace the insecurity and fear in a condition where so much is "giving way" as part of the natural change that embodies being in the world. In her own stringent and driven manner, Mrs. Pitt must do the same. Even the Angel in America must learn to embrace the condition of change as her own infallibility gives way to a reality in which she must examine what her reality is and understand it better. Even she cannot cling to that which ends up "giving way."
The drama deals with being gay in America, political modes of expression in American society, and what it means to stand on the precipice of change. All of these end up intersecting and bifurcating one another to comprise the main idea of the show. I think that it makes sense to view Harper's statement about being in the world as one of the fundamental ideas that underscore the purpose of the show. Harper might have summed up the main idea of both being in the world, but also the drama, as well.