Recently I posted some comments in the combox on Lane Keister's blog Green Baggins. (Lane Keister is a Presbyterian who pastors two congregations in North Dakota.)
In his article titled "The Church", I posted some comments here regarding who gets to determine the marks of the Church.
In his article titled "One God, One Church", I posted some comments here regarding the necessary role of magisterial authority for true ecclesial unity.
Lane also recommends Andrew Webb's article "Who Gets to Decide How the Church Should Worship?". (Andrew is a Presbyterian pastor in North Carolina.) Andrew's answer to that question is: God, as revealed in Scripture. Andrew thinks that 2 Tim 3:16 teaches that Scripture is sufficient for determining how Christians should worship. But Andrew does not explicitly say whose interpretation of Scripture is the one we should follow. Without a magisterial authority to provide an authoritative interpretation, the answer to Andrew's question is "Me".
Unity of worship (i.e. sacraments) is an essential criterion for true ecclesial unity. But we cannot have true ecclesial unity if each person is his own interpretive authority regarding worship and sacraments.
"Let unity, the greatest good of all goods, be your preoccupation." - St. Ignatius of Antioch (Letter to St. Polycarp)