I hear one common objection to moving toward what I suggest is a more biblical and healthy governance system: the question of checks and balances. If a church only has one board, and if greater authority is vested in this board, where are the checks and balances to its power?
That is a good question and one that goes to the heart of congregationalism. But it also reveals that the American church is driven more by its national polity than its biblical theology.
American government was designed as a three-part system - the legislative, judiciary and executive branches - each with different responsibilities and a carefully worked-out balance of authority so that no one branch could exert disproportionate power over the other two (at least in theory). The framers of the Constitution had a high-enough view of the depravity of man and the potential abuse of power that they tried to design national governance structures that would limit the power and, therefore, potential abuse.
Interestingly, the New Testament also provided for healthy leadership accountability, but in a different way. For instance, the New Testament always speaks of a plurality of overseers or elders, of which teaching pastors are one.
In other words, authority is never vested in an individual but in a group of leaders. In addition, strict qualifications exist for those in leadership positions, starting with character qualifications. These leaders are not at liberty to do as they please. Rather they are "under-shepherds" of Jesus, serving on His behalf, and will have to give an account for the quality and faithfulness of their ministry. That is huge accountability! Leaders are never the ultimate head of the congregation. Jesus is.
What you never find in the New Testament are competing boards of groups that exist to limit the authority of the senior leadership group, 'balance' its power or provide a check on its leadership. When we incorporate such systems into our church governance, we are modeling our systems more after our national polity than our biblical theology.
In a proper understanding of 'congregationalism' the congregation itself has the ability to override decisions of the designated leadership, but there is no biblical model or rationale in the New Testament for other checks and balances to the authority of the senior leadership.