Today I ask the same question of myself. There are some pastors that I consider to be excellent communicators of God's Word. If someone asked me why I believe Neil Plantinga, William Willimon, John Piper, and my colleagues in the Lynden CRC churches are great preachers, I would say each of those men understands not only the Gospel but the human condition.
I am far more inclined to submit to the teaching of a pastor who is not overwhelmed by the world or his own sinful desires. I realize he still sins, but yet he should still set an example for believers in speech, in life, in love, in faith and in purity. In short, he is respectable. In addition he must understand the world and all the temptations that fill it. He is neither removed from the world nor is he tossed around by the changing tides of culture and popularity.
The pastor who lives too far away from the world (I think of those in super-conservative churches or some pastors whose only responsibility is to preach) might live a pure life but he is disconnected from the realities that surround his flock. On the other hand, the pastor who attempts to relate to his congregation by constantly heralding his struggles with sin damages his credibility as a shepherd. In short, my ideal preacher is neither aloof nor a mess.
For the ultimate example of such ministry we need only look to our sinless yet soft-hearted Savior. Hebrews 4:15-16 teaches, "For we do not have a high priest who is unable to sympathize with our weaknesses, but we have one who has been tempted in every way, just as we are—yet was without sin. Let us then approach the throne of grace with confidence, so that we may receive mercy and find grace to help us in our time of need."
The issues at stake here are more than homiletical. What is the nature of Christian sanctification and holiness? How is a Christian to engage the world around him or her? Good reformed preaching models a healthy approach to life in an already-but-not-yet world. It not only delivers God's Word but it models the effect that the Gospel has had on the thoughts, actions, attitudes and words of the preacher.