The basic idea is that just as 100 years ago missionaries from the west going to peoples and cultures with no Christian background, legacy, etc. had to do a lot of study and preparatory work in order to be able to preach the gospel in an understandable way, so today we cannot assume that our neighbors have a Christian heritage giving us common ground so that we simply urge them to accept Christ.  Fewer and fewer Americans are being raised in that heritage, and large numbers of immigrants that have come to our country have backgrounds that are completely nonchristian, which is a great opportunity for us.  So being missional also means we don't just try to win people to Christ who are like us, easy to witness to, etc.  Being missional also means that we live as Christians in such a way that our lives (as individuals and as a church) commend the gospel to unbelievers.  For a more comprehensive description see this paper by Tim Keller, referenced by BTS president Dave Dunbar in a recent e-mail to BTS alumni (so far, we are in agreement with BTS on this one!).

Another good thing to keep in mind about being "missional" is that in being missional we will be displaying Christ-likeness (I think that's the point John Franke was trying to make in his video clip on BTS's web site), which is the goal of sanctification; Jesus came into the world for our salvation, acting as a servant on a mission during every moment of his earthly ministry, giving us an example.

Where we have concern is in the nature of the mission.  It appears to me that people in Emergent Village have hijacked the idea of "missional" while changing the Biblical mission; they're pulling a "bait and switch."  One can see this in Brian McLaren, who wants to shift the focus from eternity to a gospel that is effectual for the whole world before death; the mission is to rescue planet earth (more or less the old liberal social gospel).  For more discussion and documentation, see the section "A Success not of God" (success by changing the mission), in my paper "The Emerging Churches in the Book of Judges."