Some seem to have taken offense to the statement. I was going to list some of the more bizarre responses, but decided that was not as helpful-- it might have been fun, but it is still not helpful. ;-)
I'm guessing that those who were offended personalized the statement in this way: if you (personally) are not involved in global cities, you are not a serious missionary or serious about missions.
Yet, missions is a discipline, not just a personal practice. Missionaries are people engaged in a certain culture. If we are serious about missions, we will be serious about global cities. But, missionS has to take global cities seriously; missionARIES are called and serve in all kinds of contexts.
Regrettably, that's what we often do-- see the exhortation to do something to mean a devaluing of other things. Too often, we overreact to what should be an obvious statement.
Of course, part of the reality is that Twitter statements are limited in their capacity to convey the full story. So, the fault may be my inability to communicate in short bursts. Maybe I should have said, "If Christians are not serious about cities, then we are not really serious about missions."
I am comfortable with the original statement, but would also add that if you are not serious about Unreached People Groups, you are not serious about missions. And, there are other things. There are certain things that must be in any serious missions strategy, though that does not necessarily mean that every person is engaged in that mission endeavor.
Missions need to be serious about global cities (among other things); missionaries need to be serious about the people to whom God has called them.
Simply put, there are things that we need to take seriously in missions. Taking something seriously is not the same thing as being personally called to that endeavor. For example, although I was born and reared in a global city (outside NYC), I do not serve in one. Does that mean I am not serious? I hope not. But, I think any serious missions strategy must involve global cities.
There are certain things that you must hold up because they are fields that seem not to attract mission workers-- places where unreached people groups live, global cities, rural areas without access to cultural amenities, etc. Let's hold up those places, without devaluing the others. The call of God is both to a people and to a place-- some will go to rural areas, others to Unreached Peoples, still others to a suburban context. But, if you are serious about missions, you will be serious about cities.