"What's the worst thing that can happen to your direct mail campaign?"
The worst that can happen is not people complaining. It's people ignoring you. That's why above all you have the speak the language of the people you're trying to reach.
Most Pastors of contemporary churches say they do this, but we make this mistake as well. How often have you sent something out talking about your "worship"? Guess what? If you're trying to reach unchurched people, they don't know that worship is music.
We do this with our children's ministry: we essentially advertise to the kids (who can't read by the way), instead of speaking the language of the parents.
The key to being noticed is to write content that's relevant to where people are and speaks their language.