Finally, one more word about the nature of congregational authority: it’s a passive and narrow authority. It’s not the authority to lead, per se, it’s the authority to VETO bad leadership. If the elders compromise the gospel, the congregation should VETO the compromise. If a member’s life becomes compromised, the congregation should VETO (metaphorically speaking) that person’s profession of faith through excommunication. By implication, yes, I think that means the congregation also has authority to choose leaders and affirm members. But still, the mantel of day-to-day leadership and oversight falls to the elders (e.g., the congregation should seek out the elders’ leadership when it comes to new members and new leaders). The abuse of congregationalism to which MacDonald objects occurs whenever congregations try to lead: “Pastor, you need to listen to us, and do what we say!”
Remember, if God has mandated that a Kingdom of Priests be the final authority in all church matters, then he has certainly not ordained that a Kingdom of Kings operate with no leadership but their own. Within the kindom of priests there are leaders, kings if you like, who must lead, initiate, and be trusted. The question MacDonald and Leeman are asking is not whether this is the case, but rather, within an elder-led church, whether congregations should vote. In both models, congregations may interact and be brought into the decision-making process, but in Leeman's model alone, do congregations vote. I'm afraid that most of MacDonald's first post deals with congregationalism gone horribly wrong, and not with Leeman's position at all.
But again, read, think, and wrestle to be Biblical in all things!