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.