What is the main argument against the brave new world?
I see that you have tagged this with "Chapter 17" so I assume that you are asking about the argument between John the Savage and Mustapha Mond in that chapter. If so, I think that the main argument against the new society is that it takes away people's humanity.
I think that John's major argument against the brave new world is summed up pretty well in this long quote:
"You got rid of them. Yes, that's just like you. Getting rid of everything unpleasant instead of learning to put up with it. Whether 'tis better in the mind to suffer the slings and arrows of outrageous fortune, or to take arms against a sea of troubles and by opposing end them … But you don't do either. Neither suffer nor oppose. You just abolish the slings and arrows. It's too easy."
What John is saying is that the civilization has gotten rid of everything that is difficult. But it is difficult things that really make us human. Love is not easy to come by. We can have difficulties finding the right person to love and our relationships are not always easy. But things like that are what make our lives worth living.
So the major argument against the society is that it has destroyed everything that made life worth living.
The "brave new world" described by Mustapha Mond has done away with God and religion as traditionally understood, and along with that the moral virtues of patience in suffering, sacrifice, deep caring for others, courage, and risk taking. It has eradicated real literature and art in favor of mindless entertainment, and by filling all day, every day with communal activity has denied people the opportunity of solitude, contemplation and self awareness. Security and material comfort have replaced freedom and the chance for the human beings to develop creativity and moral character. Mond defends this, saying that fashioning a hygienic world where people never have to face anything unpleasant or confront all but the most shallow emotions renders religion, literature and moral courage unnecessary. But John the Savage longs for living at the soul level, in a cuture in which he is challenged to become a better self. As he puts it: “But I don’t want comfort. I want God, I want poetry, I want real danger, I want freedom, I want goodness. I want sin.”
The main argument against this engineered "brave new" society is that it robs people of their souls and their deepest humanity.
The main argument against the brave new world, as stated by John the Savage to Mustapha Mond in chapter 17, is the right to be unhappy, which is synonymous with the right to be imperfectly human. In Mustapha Mond's manufactured, comfortable society, where every citizen is perfectly content at all times because soma solves everything, John the Savage argues the right to experience life as an imperfect human. John the Savage's argument focuses on the opportunities citizens should have to experience doubt, spontaneity, uncertainty, emotions, and faith, which are essential elements to being imperfectly human. Without genuinely experiencing discomfort, humanity is void of purpose and meaning, which is emphasized by the fact that the concept of God does not exist in the Mustapha Mond's modern society. When the Controller argues that there is no need for a civilized human to experience anything unpleasant, John the Savage responds by asking, "Isn’t there something in living dangerously?" (Huxley, 163). Essentially, John the Savage is arguing for the right to be imperfectly human and the opportunity to experience the visceral sensations of an unpredictable, dangerous life.