Well, part of this would be your reasoning: what do you think are the most important things?
In most people's list, though, would be the core elements of the book:
This is a book about an ideal society.
It is a critique of our society--and a praise of our society.
It emphasizes equality through conformity.
In the book, special gifts produce special responsibilities and opportunities.
In the end, individual freedom and experience is crucial.
I'd start there.