Each person alienates, by the social compact, only that portion of his power, his goods, and liberty whose use is of consequence to the community; but we must also grant that only the sovereign is the judge of what is of consequence.
Gist of Idea
We alienate to society only what society needs - but society judges that, not us
Jean-Jacques Rousseau (The Social Contract (tr Cress) , II.04)
The weakness here is how society sees its needs. He seems to assume that two societies will arrive at almost identical general wills, but Spartans, Prussians and Serbs may require the lives of your children for the state.
Rousseau,Jean-Jacques: 'The Basic Political Writings', ed/tr. Cress,Donald A. [Hackett 1987], p.157