If a church has over 50 people in it, I dislike interaction from the front. A little bit is fine. But that sort of "Hi everybody!" "Morning Mikey" sort of thing is tacky, I think.
I similarly think that breaking up into small groups to discuss things should be kept to a minimum. I have come to the public gathering of the church to engage in large-group activities, I'm not in the frame of mind for small-group activities. I find it a bit invasive.
