One of the features of the American church is the historical focus on the large "tent meeting" atmosphere for life-changing decisions. At many churches, every worship service ends with an "alter call" wherein people are invited to declare their decisions publicly. While there is a place for this kind of thing, my experience has been that the vast majority of serious, vital Christian growth occurs primarily in smaller groups, where the Scriptural teaching, rebuking, correcting, and training in righteousness really takes hold. Those who don't take part largely remain stagnant.
John Piper exhorts those who don't meet in small groups: "You are cutting yourself off from extraordinary blessings."
Piper further illustrates the point in another message on how it is in small gatherings that we have our "Esther Moments", when God challenges us to take large and life-changing steps of faith.
The text for the entire message on "Esther Moments" can be found here.