What is the most powerful element of Julia's careless nudity on page 31

First, may I suggest that you ask questions with chapter numbers rather than page numbers since we may not have the same editions of the book.

I am assuming that you are talking about the part in Chapter 3 where Winston dreams about the girl who seems very much like Julia (although he does not yet know Julia at this point).

In that dream, the most important aspect of the girl's nudity is the complete freedom with which she acts.  She does not act all self-conscious like everyone else in the society.  Instead, she seems to be able to do whatever she wants.  This is a very powerful thing in a society where no one is able to just do what they want in a carefree manner.