The church is a gospel centered community on mission. It is not a social club, a political party, a musical appreciation society or a self-help group. It is a group of disciple-making disciples united by the transforming message of the gospel. How should this affect the preaching?
Both Christians and non-Christians need to hear the gospel, so preaching the gospel should not be something that is reserved for special occasions. The sermon should be biblical, not just motivational speech or a "thought for the day". Those listening should hear the message of grace not just advice about sorting your life out.
The sermon should not just be an individual hearing a load of information, that could be done at home. Community helps us to access what has been preached, through discussion, debate, encouragement, challenge and action. Whenever I hear someone talking about how they were deeply affected by a particular sermon, it always seems a lot better than I remembered it!
Finally, a missional attitude should shape the sermon, addressing the issues that non-Christians may have as well as using accessible language.