Lately I've heard how Tim Keller does not like how people attribute Redeemer's success to him. I can understand if he's trying to be modest, and that he's pointing all credits to God alone.
However, there is an easy way to see this clearly:
Tim Keller's ministry was unique in a way that it was obvious that he depended his journey entirely on God - that he moved to New York not because he wanted to see success, for he acknowledged that it could be God's calling for him to fail in New York.
However, I cannot say the same for most other pastoral staffs at Redeemer. It would be easier to say that they joined Redeemer not because they acknowledged that God could be calling them to fail in their pastoral career. Redeemer was already flourishing. It is unlikely that they would dare found a church.
One might argue for "different callings". I can second that, half way. For the point is, why Tim Keller was important to Redeemer. All of these, nonetheless, are attributed to God. It would be like saying Time Square Church's loss of their senior pastor David Wilkerson has made the church rather directionless, despite the ongoing popularity. But it was a Charismatic church becoming a worse Charismatic church. As for Redeemer, a presbyterian church, according to current progress, if without Tim Keller suddenly, the change would be far more drastic. Some may say: It still works! as Time Square Church, but I beg to differ, because I believe the state of Time Square Church is not the same as it used to. I could be wrong, but I once sat in Time Square church seeing so many loving God's words, but on my last visit, I experienced more entertainment than instilling a heart for God.
The other thing is that Tim Keller's sermons are more on the expository side, while the rest of the pastors' are topical.