In church services that make the sermon the big and only major event in the order of worship, the pastor has a huge responsibility. The sermon must attempt to do what has been traditionally accomplished in the liturgy itself. Services that “build up” to the sermon with a few hymns, some praying, and a few announcements leave the people needing something that must be given by the pastor in the sermon.
Having jettisoned a “traditional” order of service, one that includes confession of sin and absolution, we must now train pastors to take the congregation through the entire process of covenant renewal in the sermon. And so many Evangelical Presbyterian pastors are judged severely for not "preaching the Gospel" because they don’t lead people through a process of calling them to confession and trusting in Jesus in every sermon. This is what is often meant by the demand that every sermon must be "Christ centered, " and all that.
I am sympathetic to this. In churches that have no confession, forgiveness, and no culmination at the Lord’s Table, if the preacher does not give the people the Gospel in the sermon they would never hear it or experience it in the Sunday service. So preachers have to make sure that people feel their sin and guilt, then hear the gospel, then be told what to do, etc. And since the “non-traditional” service does not end with the Lord's Supper, there will even be some substitute "peace" that the preacher must seek to establish in the people's hearts before the end of the sermon.
A sermon in the middle of a full-bodied covenant renewal worship service, however, can be something entirely different. The pastor is free to preach on a text that may not be amenable to the kind of Gospel message that might otherwise be demanded in a typical spartan service. Since the people have been called to confession and have been forgiven in Christ before the sermon begins, the sermon itself may be simply instruction from God’s Word. And the pastor need not feel any guilt that he hasn’t “preached the Gospel.”
But people are slow to buy into this. Mostly because most churches still don't have the Gospel embodied in their liturgy. But also because homiletics and worship are two entirely different classes in seminary. Sigh.