The danger that unrestrained majority rule will oppress minorities or individuals, whether through law or through social pressure to conform.
In more detail
Madison treated the problem as that of majority faction in Federalist No. 10. Tocqueville devoted a chapter of Democracy in America to the tyranny of the majority, and Mill extended the worry to social conformity in On Liberty. Constitutional rights, separation of powers, federalism and supermajority rules are common responses.