How does Edwards's reputation as a brilliant spiritual leader make this sermon more effective?

A speaker's credibility, or ethos, is important when the aim of the speech or sermon is to persuade.  Edwards delivered the sermon "Sinners in the Hands of an Angry God" to a congregation in Enfield, CT as a visiting minister--and one with a formidable reputation.  Edwards was a theologian, which means that he studied religion as an academic discipline.  He began studying at Yale in 1716 when he was just twelve years old, and besides religion, he was interested in natural philosophy.  By all accounts, his intellect was tremendous.
Edwards began preaching in 1727 as an assistant to his grandfather, and then took over the congregation at Northampton, Massachusetts when his grandfather died in 1729. By the time he gave the famous sermon in the midst of the Great Awakening in 1741, Edwards had been studying religion for twenty-five years and was in demand as a sometimes controversial but highly regarded Reform Congregationalist minister.
It is reasonable to assume that a minister with a lesser pedigree might not have been able to pull off a sermon as severe as "Sinners in the Hands of an Angry God."  Edwards went on at length about God's wrath, the horrors of Hell, and how dangerous complacency was when it came to one's eternal afterlife.