I think this quote from the article helps describe one way that he has such an impact, in my life and in the lives of others:
"White believes Keller's unique gift is to preach to both Christians and non-Christians in the same terms, without making a choice between evangelism and discipleship: "Tim uses the gospel surgically on the heart. The gospel is what we need to come to faith and also what we need to grow." A theology of grace uses the same language to challenge both the runaway son and the solid older brother."The article also mentions the other significant ideas that have been very influential, namely the subject of idolatry as well as the importance of the city.
Read the whole thing