If you surveyed 1000 people throughout the last 2000 years of history, I'm guessing that the #1 question that group would have for Jesus is, "why did you give power to your church to retain sins?"
What this difficult bit of text seems to be saying, as far as I can tell, is mostly lost in translation. It would be better to read it (and Bill please tell me if I'm wrong), "If y'all forgive the sins of any - they have already been forgiven. If you retain the sins of any - they have already been retained."
So the job of the Church, then, becomes not the judge of what sin is forgivable and what sin is not, but instead the job of the Church is to discern whom God has forgiven and who He has not. From my experience the former will include everyone and the latter, well the only people who God does not forgive are those who do not themselves forgive. Remember that old prayer, "forgive us our trespasses as we forgive those who trespass against us."
This small piece of scripture has been abused, a lot, and for that my heart aches. But to ignore it because of its past is to allow that abuse to continue. As preachers, we must work to find the spirit of the text and then educate our congregations. We can't change the whole Church, but we can be faithful in our little corner of it.