It's a paradox. Parishioners often cite the sermon as the pinnacle of the worship service yet they retain little from it.
People enter the church with the societal expectation that they'll be preached at .
Hence, a good sermon is anything that makes that 20 or 30 minutes pass with a minimum of discomfort. We've trained people to seek comfort. The problem lies with the medium itself. Typically it's presented in the traditional teaching mode - straight lecture. Straight speaking to a crowd results little retention and little learning.